* Filesystem corruption on RAID1 @ 2017-07-13 15:35 Gionatan Danti 2017-07-13 16:48 ` Roman Mamedov 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-07-13 15:35 UTC (permalink / raw) To: linux-raid; +Cc: g.danti Hi list, today I had an unexpected filesystem corruption on a RAID1 machine used for backup purposes. I would like to reconstruct what possibly happened on why, so I am asking for your help. System specs: - OS CentOS 7.2 x86_64 with kernel 3.10.0-514.6.1.el7.x86_64 - 2x SEAGATE ST4000VN000-1H4168 (4 TB 5900rpm disks) - 4 GB DDR3 RAM - Intel(R) Pentium(R) CPU G3260 @ 3.30GHz Today, I found the machine crashed with an XFS warning about corrupted metadata. The warning stated that in-core (or in-memory) data corruption was detected so, thinking about a DRAM-related problem (no ECC memory on this small box...) I simply rebooted tha machine. To no avail - the same problem immediately happened, preventing the machine from booting (the root filesystem did not mount). After the filesystem was repaired (with significant corruption signs, also due to the clearing of the XFS journal), I looked at dmesg and found something interesting: a raid-resync action was *automatically* performed, as when re-attaching a (detached) disk. I start investigating in /var/log/messages and found plenty of these errors, spanning many days: ... Jul 10 03:24:01 nas kernel: ata1.00: failed command: READ FPDMA QUEUED Jul 10 14:50:54 nas kernel: ata1.00: failed command: FLUSH CACHE EXT Jul 12 03:14:41 nas kernel: ata1.00: failed command: WRITE FPDMA QUEUED ... To me, it seems that a disks (the first one, sda) had problem executing some SATA commands, becoming out-of-sync from the second one (sdb). However it was not kicked out the array, as both /var/log/messages *and* my custom monitoring script (which keep an eye on /proc/mdstat) reported nothing. Moreover, inspecting both the SMART values and log show *no* error at all. Question 1: it is possible to have such a situation, where a failed command *silently* put the array in out-of-sync state? At a certain point, the machine crashed. I noticed and rebooted it. Question 2: it is possible that the old disk become offline just before the crash and, by rebooting, the mdadm re-added it to the array? Question 3: if so, it is possible that the corruption was due to the first disk being the one read by the md array and, by extension, by the filesystem? Any thoughts will be greatly appreciated. Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 15:35 Filesystem corruption on RAID1 Gionatan Danti @ 2017-07-13 16:48 ` Roman Mamedov 2017-07-13 21:28 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Roman Mamedov @ 2017-07-13 16:48 UTC (permalink / raw) To: Gionatan Danti; +Cc: linux-raid On Thu, 13 Jul 2017 17:35:12 +0200 Gionatan Danti <g.danti@assyoma.it> wrote: > Jul 10 03:24:01 nas kernel: ata1.00: failed command: READ FPDMA QUEUED Failed reads are not as bad, as they are just retried. > Jul 12 03:14:41 nas kernel: ata1.00: failed command: WRITE FPDMA QUEUED But these WILL cause incorrect data written to disk, in my experience. After that, one of your disks will contain some corruption, whether in files, or (as you discovered) in the filesystem itself. mdadm may or may not read from that disk, as it chooses the mirror for reads pretty much randomly, using the least loaded one. And even though the other disk still contains good data, there is no mechanism for the user-space to say "hey, this doesn't look right, what's on the other mirror?" Check your cables and/or disks themselves. If you know that only one disk had these write errors all the time, you could try disconnecting it from mirror, and checking if you can get a more consistent view of the filesystem on the remaining one. P.S: about my case (which I witnessed on a RAID6): * copy a file to the array, one disk will hit tons of WRITE FPDMA QUEUED errors (due to insufficient power and/or bad data cable). * the file that was just copied, turns out to be corrupted when reading back. * the problem disk WILL NOT get kicked from the array during this. -- With respect, Roman ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 16:48 ` Roman Mamedov @ 2017-07-13 21:28 ` Gionatan Danti 2017-07-13 21:34 ` Reindl Harald 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-07-13 21:28 UTC (permalink / raw) To: Roman Mamedov; +Cc: linux-raid, g.danti Il 13-07-2017 18:48 Roman Mamedov ha scritto: > > Failed reads are not as bad, as they are just retried. > I agree, I reported them only to give a broad picture of the system state :) >> Jul 12 03:14:41 nas kernel: ata1.00: failed command: WRITE FPDMA >> QUEUED > > But these WILL cause incorrect data written to disk, in my experience. > After > that, one of your disks will contain some corruption, whether in files, > or (as > you discovered) in the filesystem itself. This is the "scary" part: if the write was not acknowledged as committed to disk, why the block layer did not report it to the MD driver? Or if the block layer reported that, why MD did not kick the disk out of the array? > mdadm may or may not read from that > disk, as it chooses the mirror for reads pretty much randomly, using > the least > loaded one. And even though the other disk still contains good data, > there is > no mechanism for the user-space to say "hey, this doesn't look right, > what's > on the other mirror?" I understand and agree with that. I'm fully aware that MD can not (by design) detect/correct corrupted data. However, I wonder if, and why, a disk with obvious errors was not kicked out of the array. > > Check your cables and/or disks themselves. > I tried reseating and inverting the cables ;) Let see if the problem disappears or if it "follow" the cable/drive/interface... > If you know that only one disk had these write errors all the time, you > could > try disconnecting it from mirror, and checking if you can get a more > consistent view of the filesystem on the remaining one. > > P.S: about my case (which I witnessed on a RAID6): > > * copy a file to the array, one disk will hit tons of WRITE FPDMA > QUEUED > errors (due to insufficient power and/or bad data cable). > * the file that was just copied, turns out to be corrupted when > reading back. > * the problem disk WILL NOT get kicked from the array during this. Wow, a die-hard data corruption. It seems VERY similar to what happened to me, and the key problem seems the same: a failing drive was not detached from the array in a timely fashion. Thanks very much for reporting, Roman. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 21:28 ` Gionatan Danti @ 2017-07-13 21:34 ` Reindl Harald 2017-07-13 22:34 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Reindl Harald @ 2017-07-13 21:34 UTC (permalink / raw) To: Gionatan Danti, Roman Mamedov; +Cc: linux-raid Am 13.07.2017 um 23:28 schrieb Gionatan Danti: > I understand and agree with that. I'm fully aware that MD can not (by > design) detect/correct corrupted data. However, I wonder if, and why, a > disk with obvious errors was not kicked out of the array. maybe because the disk is, well, not in a good shape and don't know that by itself - i had storage devices which refused to write but said nothing (flash media), frankly you where able to even format that crap and overwrite if with zeros and all looked fine - until you pulled the broken device and inserted it again - same data as yesterday - a sd-card doestroyed a smartphone phisically by empty the whole battey within 30 minutes while sitting in the cinema broken hardware don't know that it's broken moste of the time that#s why you need always backups or can just delete the data at all because they are not important thins like above only could be detected by verify every write with an uncached read/verify which would lead in a uneccaptable performane penalty (and no filesystems with checksums won't magically recover your data, they just tell you realier they are gone) ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 21:34 ` Reindl Harald @ 2017-07-13 22:34 ` Gionatan Danti 2017-07-14 0:32 ` Reindl Harald 2017-07-14 1:48 ` Chris Murphy 0 siblings, 2 replies; 46+ messages in thread From: Gionatan Danti @ 2017-07-13 22:34 UTC (permalink / raw) To: Reindl Harald; +Cc: Roman Mamedov, linux-raid, g.danti Il 13-07-2017 23:34 Reindl Harald ha scritto: > maybe because the disk is, well, not in a good shape and don't know > that by itself > But the kernel *does* know that, as the dmesg entries clearly show. Basically, some SATA commands timed-out and/or were aborted. As the kernel reported these erros in dmesg, why do not use these information to stop a failing disk? > > (and no filesystems with checksums won't magically recover > your data, they just tell you realier they are gone) > Checksummed filesystem that integrates their block-level management (read: ZFS or BTRFS) can recover the missing/corrupted data by the healthy disks, discarging corrupted data based on the checksum mismatch. Anyway, this has nothing to do with linux software RAID. I was only "thinking loud" :) Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 22:34 ` Gionatan Danti @ 2017-07-14 0:32 ` Reindl Harald 2017-07-14 0:52 ` Anthony Youngman 2017-07-14 10:46 ` Gionatan Danti 2017-07-14 1:48 ` Chris Murphy 1 sibling, 2 replies; 46+ messages in thread From: Reindl Harald @ 2017-07-14 0:32 UTC (permalink / raw) To: Gionatan Danti; +Cc: Roman Mamedov, linux-raid Am 14.07.2017 um 00:34 schrieb Gionatan Danti: > Il 13-07-2017 23:34 Reindl Harald ha scritto: >> maybe because the disk is, well, not in a good shape and don't know >> that by itself > > But the kernel *does* know that, as the dmesg entries clearly show. > Basically, some SATA commands timed-out and/or were aborted. As the > kernel reported these erros in dmesg, why do not use these information > to stop a failing disk? because you won't be that happy when the kernel spits out a disk each time a random SATA command times out - the 4 RAID10 disks on my workstation are from 2011 and showed them too several times in the past while they are just fine here you go: http://strugglers.net/~andy/blog/2015/11/09/linux-software-raid-and-drive-timeouts/ ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 0:32 ` Reindl Harald @ 2017-07-14 0:52 ` Anthony Youngman 2017-07-14 1:10 ` Reindl Harald 2017-07-14 10:46 ` Gionatan Danti 1 sibling, 1 reply; 46+ messages in thread From: Anthony Youngman @ 2017-07-14 0:52 UTC (permalink / raw) To: Reindl Harald, Gionatan Danti; +Cc: Roman Mamedov, linux-raid On 14/07/17 01:32, Reindl Harald wrote: > > > Am 14.07.2017 um 00:34 schrieb Gionatan Danti: >> Il 13-07-2017 23:34 Reindl Harald ha scritto: >>> maybe because the disk is, well, not in a good shape and don't know >>> that by itself >> >> But the kernel *does* know that, as the dmesg entries clearly show. >> Basically, some SATA commands timed-out and/or were aborted. As the >> kernel reported these erros in dmesg, why do not use these information >> to stop a failing disk? > > because you won't be that happy when the kernel spits out a disk each > time a random SATA command times out - the 4 RAID10 disks on my > workstation are from 2011 and showed them too several times in the past > while they are just fine > Except, in the context of this thread, the alternative is CORRUPTED DATA. I certainly know which one I would prefer, and that is a crashed array! If a *write* fails, then a failed array may well be the least of the user's problems - and silent failure merely makes matters worse! I know, the problem is that linux isn't actually that good at propagating errors back to user space, and I believe that's a fault of POSIX. So fixing the problem might be a massive job - indeed I think it is. But that's no excuse for mocking someone just because they want to be told that the system has just gone and lost their work for them ... Oh - and isn't that what raid is *supposed* to do? Kick a disk on a write failure? Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 0:52 ` Anthony Youngman @ 2017-07-14 1:10 ` Reindl Harald 0 siblings, 0 replies; 46+ messages in thread From: Reindl Harald @ 2017-07-14 1:10 UTC (permalink / raw) To: Anthony Youngman, Gionatan Danti; +Cc: Roman Mamedov, linux-raid Am 14.07.2017 um 02:52 schrieb Anthony Youngman: > On 14/07/17 01:32, Reindl Harald wrote: >> >> >> Am 14.07.2017 um 00:34 schrieb Gionatan Danti: >>> Il 13-07-2017 23:34 Reindl Harald ha scritto: >>>> maybe because the disk is, well, not in a good shape and don't know >>>> that by itself >>> >>> But the kernel *does* know that, as the dmesg entries clearly show. >>> Basically, some SATA commands timed-out and/or were aborted. As the >>> kernel reported these erros in dmesg, why do not use these >>> information to stop a failing disk? >> >> because you won't be that happy when the kernel spits out a disk each >> time a random SATA command times out - the 4 RAID10 disks on my >> workstation are from 2011 and showed them too several times in the >> past while they are just fine >> > Except, in the context of this thread, the alternative is CORRUPTED > DATA. I certainly know which one I would prefer, and that is a crashed > array! > > If a *write* fails, then a failed array may well be the least of the > user's problems - and silent failure merely makes matters worse! i doubt that you would repeat that if for whatever load condition a random SATA timeout occours on both disks of a mirror and you lose some TB of data while in *that case* not silent corruption or anything else bad would have happened except a short lag did you really read http://strugglers.net/~andy/blog/2015/11/09/linux-software-raid-and-drive-timeouts/ or just ignored it on purpose? > I know, the problem is that linux isn't actually that good at > propagating errors back to user space, and I believe that's a fault of > POSIX. So fixing the problem might be a massive job - indeed I think it is. > > But that's no excuse for mocking someone just because they want to be > told that the system has just gone and lost their work for them ... nobody is mocking someone, i just explained why things are not as simple as they appear and with a 2 disk mirror they are always complicated in any error case by lack of quorum > Oh - and isn't that what raid is *supposed* to do? Kick a disk on a > write failure? if things only would be that easy in the real world... in doubt with a mirrored RAID without data checksums *you have no way* to guarantee what is the correct data if something flips and "Except, in the context of this thread" is nice but won't help in general and trying to handle each and every bordercase with some workarounds would lead nowhere yes, agreed, silent corruption is bad, hardware lying about data written is bad, but if things would be that easy all that won't happen and nobody would have spent time for develop checksummed filesystems ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 0:32 ` Reindl Harald 2017-07-14 0:52 ` Anthony Youngman @ 2017-07-14 10:46 ` Gionatan Danti 2017-07-14 10:58 ` Reindl Harald 2017-08-17 8:23 ` Gionatan Danti 1 sibling, 2 replies; 46+ messages in thread From: Gionatan Danti @ 2017-07-14 10:46 UTC (permalink / raw) To: Reindl Harald; +Cc: Roman Mamedov, linux-raid, g.danti Il 14-07-2017 02:32 Reindl Harald ha scritto: > because you won't be that happy when the kernel spits out a disk each > time a random SATA command times out - the 4 RAID10 disks on my > workstation are from 2011 and showed them too several times in the > past while they are just fine > > here you go: > http://strugglers.net/~andy/blog/2015/11/09/linux-software-raid-and-drive-timeouts/ Hi, so a premature/preventive drive detachment is not a silver bullet, and I buy it. However, I would at least expect this behavior to be configurable. Maybe it is, and I am missing something? Anyway, what really surprise me is *not* the drive to not be detached, rather permitting that corruption make its way into real data. I naively expect that when a WRITE_QUEUED or CACHE_FLUSH command aborts/fails (which *will* cause data corruption if not properly handled) the I/O layer has the following possibilities: a) retry the write/flush. You don't want to retry indefinitely, so the kernel need some type of counter/threshold; when the counter is reached, continue with b). This would mask out sporadic errors, while propagating recurring ones; b) notify the upper layer that a write error happened. For synchronized and direct writes it can notify that by simply returning the correct exit code to the calling function. In this case, the block layer should return an error to the MD driver, which must act accordlying: for example, dropping the disk from the array. c) do nothing. This seems to me by far the worst choice. If b) is correcly implemented, it should prevent corruption to accumulate on the drives. Please also note the *type* of corrupted data: not only user data, but filesystem journal and metadata also. The latter should be protected by the using of write barriers / FUAs, so they should be able to stop themselves *before* corruption. So I have some very important questions: - how does MD behave when flushing data to disk? - does it propagate write barriers? - when a write barrier fails, is the error propagated to the upper layers? Thanks you all. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 10:46 ` Gionatan Danti @ 2017-07-14 10:58 ` Reindl Harald 2017-08-17 8:23 ` Gionatan Danti 1 sibling, 0 replies; 46+ messages in thread From: Reindl Harald @ 2017-07-14 10:58 UTC (permalink / raw) To: Gionatan Danti; +Cc: Roman Mamedov, linux-raid Am 14.07.2017 um 12:46 schrieb Gionatan Danti: > Il 14-07-2017 02:32 Reindl Harald ha scritto: >> because you won't be that happy when the kernel spits out a disk each >> time a random SATA command times out - the 4 RAID10 disks on my >> workstation are from 2011 and showed them too several times in the >> past while they are just fine >> >> here you go: >> http://strugglers.net/~andy/blog/2015/11/09/linux-software-raid-and-drive-timeouts/ >> > > Hi, so a premature/preventive drive detachment is not a silver bullet, > and I buy it. However, I would at least expect this behavior to be > configurable. Maybe it is, and I am missing something? dunno, maybe it is, but it wouldn't be wise because in case of a RAID5 rebuilding after a disk-failure would become even more dangerous on a large array as it is already > Anyway, what really surprise me is *not* the drive to not be detached, > rather permitting that corruption make its way into real data. I naively > expect that when a WRITE_QUEUED or CACHE_FLUSH command aborts/fails > (which *will* cause data corruption if not properly handled) the I/O > layer has the following possibilities: given that i have seen at least SD-cards confirming over hours sucessful writes with no single error in the syslog maybe it was one of the rare cases where the hardware lied and if that is the case you have nearly no chance on the software layer except verify each write with a uncached read of the block which would have a unacceptable impact on performance ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 10:46 ` Gionatan Danti 2017-07-14 10:58 ` Reindl Harald @ 2017-08-17 8:23 ` Gionatan Danti 2017-08-17 12:41 ` Roger Heflin 1 sibling, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-17 8:23 UTC (permalink / raw) To: Reindl Harald; +Cc: Roman Mamedov, linux-raid, Gionatan Danti On 14/07/2017 12:46, Gionatan Danti wrote:> Hi, so a premature/preventive drive detachment is not a silver bullet, > and I buy it. However, I would at least expect this behavior to be > configurable. Maybe it is, and I am missing something? > > Anyway, what really surprise me is *not* the drive to not be detached, > rather permitting that corruption make its way into real data. I naively > expect that when a WRITE_QUEUED or CACHE_FLUSH command aborts/fails > (which *will* cause data corruption if not properly handled) the I/O > layer has the following possibilities: > > a) retry the write/flush. You don't want to retry indefinitely, so the > kernel need some type of counter/threshold; when the counter is reached, > continue with b). This would mask out sporadic errors, while propagating > recurring ones; > > b) notify the upper layer that a write error happened. For synchronized > and direct writes it can notify that by simply returning the correct > exit code to the calling function. In this case, the block layer should > return an error to the MD driver, which must act accordlying: for > example, dropping the disk from the array. > > c) do nothing. This seems to me by far the worst choice. > > If b) is correcly implemented, it should prevent corruption to > accumulate on the drives. > > Please also note the *type* of corrupted data: not only user data, but > filesystem journal and metadata also. The latter should be protected by > the using of write barriers / FUAs, so they should be able to stop > themselves *before* corruption. > > So I have some very important questions: > - how does MD behave when flushing data to disk? > - does it propagate write barriers? > - when a write barrier fails, is the error propagated to the upper layers? > > Thanks you all. > Hi all, having some free time, I conducted some new tests and I am now able to 100% replicate the problem. To recap: a filesystem on a RAID1 array was corrupted due to SATA WRITEs failing but *no* I/O error being reported to higher layer (ie: mdraid/mdadm). I already submitted my findings on the linux-scsi mailing list, but I want to share them here because they can be useful to others. On the affected machine, /var/log/messages shown some "failed command: WRITE FPDMA QUEUED" entries, but *no* action (ie: kick off disk) was taken by MDRAID. I tracked down the problem to an instable power supply (switching power rail/connector solved the problem). In the latest day I had some spare time and I am now able to regularly replicate the problem. Basically, when a short powerloss happens, the scsi midlayer logs some failed operations, but does *not* pass these errors to higher layer. In other words, no I/O error is returned to the calling application. This is the reason why MDRAID did not kick off the instable disk on the machine with corrupted filesystem. To replicated the problem, I wrote a large random file on a small MD RAID1 array, pulling off the power of one disk from about 2 seconds. The file write operation stopped for some seconds, than recovered. Running an array check resulted in a high number of mismatch_cnt sectors. Dmesg logged the following lines: Aug 16 16:04:02 blackhole kernel: ata6.00: exception Emask 0x50 SAct 0x7fffffff SErr 0x90a00 action 0xe frozen Aug 16 16:04:02 blackhole kernel: ata6.00: irq_stat 0x00400000, PHY RDY changed Aug 16 16:04:02 blackhole kernel: ata6: SError: { Persist HostInt PHYRdyChg 10B8B } Aug 16 16:04:02 blackhole kernel: ata6.00: failed command: WRITE FPDMA QUEUED Aug 16 16:04:02 blackhole kernel: ata6.00: cmd 61/00:00:10:82:09/04:00:00:00:00/40 tag 0 ncq 524288 out#012 res 40/00:d8:10:72:09/00:00:00:00:00/40 Emask 0x50 (ATA bus error) Aug 16 16:04:02 blackhole kernel: ata6.00: status: { DRDY } ... Aug 16 16:04:02 blackhole kernel: ata6.00: failed command: WRITE FPDMA QUEUED Aug 16 16:04:02 blackhole kernel: ata6.00: cmd 61/00:f0:10:7e:09/04:00:00:00:00/40 tag 30 ncq 524288 out#012 res 40/00:d8:10:72:09/00:00:00:00:00/40 Emask 0x50 (ATA bus error) Aug 16 16:04:02 blackhole kernel: ata6.00: status: { DRDY } Aug 16 16:04:02 blackhole kernel: ata6: hard resetting link Aug 16 16:04:03 blackhole kernel: ata6: SATA link down (SStatus 0 SControl 310) Aug 16 16:04:04 blackhole kernel: ata6: hard resetting link Aug 16 16:04:14 blackhole kernel: ata6: softreset failed (device not ready) Aug 16 16:04:14 blackhole kernel: ata6: hard resetting link Aug 16 16:04:24 blackhole kernel: ata6: softreset failed (device not ready) Aug 16 16:04:24 blackhole kernel: ata6: hard resetting link Aug 16 16:04:35 blackhole kernel: ata6: link is slow to respond, please be patient (ready=0) Aug 16 16:04:42 blackhole kernel: ata6: SATA link down (SStatus 0 SControl 310) Aug 16 16:04:46 blackhole kernel: ata6: hard resetting link Aug 16 16:04:46 blackhole kernel: ata3: exception Emask 0x10 SAct 0x0 SErr 0x40d0202 action 0xe frozen Aug 16 16:04:46 blackhole kernel: ata3: irq_stat 0x00400000, PHY RDY changed Aug 16 16:04:46 blackhole kernel: ata3: SError: { RecovComm Persist PHYRdyChg CommWake 10B8B DevExch } Aug 16 16:04:46 blackhole kernel: ata3: hard resetting link Aug 16 16:04:51 blackhole kernel: ata3: softreset failed (device not ready) Aug 16 16:04:51 blackhole kernel: ata3: applying PMP SRST workaround and retrying Aug 16 16:04:51 blackhole kernel: ata3: SATA link up 3.0 Gbps (SStatus 123 SControl 300) Aug 16 16:04:51 blackhole kernel: ata3.00: configured for UDMA/133 Aug 16 16:04:51 blackhole kernel: ata3: EH complete Aug 16 16:04:52 blackhole kernel: ata6: softreset failed (device not ready) Aug 16 16:04:52 blackhole kernel: ata6: applying PMP SRST workaround and retrying Aug 16 16:04:52 blackhole kernel: ata6: SATA link up 1.5 Gbps (SStatus 113 SControl 310) Aug 16 16:04:52 blackhole kernel: ata6.00: configured for UDMA/133 Aug 16 16:04:52 blackhole kernel: ata6: EH complete As you can see, while failed SATA operation were logged in dmesg (and /var/log/messages), no I/O errors where returned to the upper layer (MDRAID) or the calling application. I had to say that I *fully expect* some inconsistencies: after all, removing the power wipes the volatile disk's DRAM cache, which means data loss. However, I really expected some I/O errors to be thrown to the higher layers, causing visible reactions (ie: a disks pushed out the array). With no I/O errors returned, the higher layer application are effectively blind. Moreover, both disks show the *same RAID event number*, so the MDRAID layer can not automatically offline/don't read the corrupted disk. Here is the relevant output: [root@blackhole storage]# mdadm -E /dev/sd[bc]1 /dev/sdb1: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 8d06acd3:95920a78:069a7fc3:a526ca8a Name : blackhole.assyoma.it:200 (local to host blackhole.assyoma.it) Creation Time : Wed Aug 16 15:11:14 2017 Raid Level : raid1 Raid Devices : 2 Avail Dev Size : 2095104 (1023.00 MiB 1072.69 MB) Array Size : 1047552 (1023.00 MiB 1072.69 MB) Data Offset : 2048 sectors Super Offset : 8 sectors Unused Space : before=1960 sectors, after=0 sectors State : clean Device UUID : 97bfbe06:89016508:2cb250c9:937a5c2e Update Time : Thu Aug 17 10:09:28 2017 Bad Block Log : 512 entries available at offset 72 sectors Checksum : 52670329 - correct Events : 759 Device Role : Active device 0 Array State : AA ('A' == active, '.' == missing, 'R' == replacing) /dev/sdc1: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 8d06acd3:95920a78:069a7fc3:a526ca8a Name : blackhole.assyoma.it:200 (local to host blackhole.assyoma.it) Creation Time : Wed Aug 16 15:11:14 2017 Raid Level : raid1 Raid Devices : 2 Avail Dev Size : 2095104 (1023.00 MiB 1072.69 MB) Array Size : 1047552 (1023.00 MiB 1072.69 MB) Data Offset : 2048 sectors Super Offset : 8 sectors Unused Space : before=1960 sectors, after=0 sectors State : clean Device UUID : bf660182:701430fd:55f5fde9:6ded709e Update Time : Thu Aug 17 10:09:28 2017 Bad Block Log : 512 entries available at offset 72 sectors Checksum : 5336733f - correct Events : 759 Device Role : Active device 1 Array State : AA ('A' == active, '.' == missing, 'R' == replacing) More concerning is the fact that these undetected errors can make their way even when the higher application consistently calls sync() and/or fsync. In other words, it seems than even acknowledged writes can fail in this manner (and this is consistent with the first machine corrupting its filesystem due to journal trashing - XFS journal surely uses sync() where appropriate). The mechanism seems the following: - an higher layer application issue sync(); - a write barrier is generated; - a first FLUSH CACHE command is sent to the disk; - data are written to the disk's DRAM cache; - power is lost! The volatile cache lose its content; - power is re-established and the disk become responsive again; - a second FLUSH CACHE command is sent to the disk; - the disk acks each SATA command, but real data are lost. As a side note, when the power loss or SATA cable disconnection is relatively long (over 10 seconds, as by eh timeout), the SATA disks become disconnected (and the MD layer acts accordlying): Aug 16 16:12:20 blackhole kernel: ata6.00: exception Emask 0x50 SAct 0x7fffffff SErr 0x490a00 action 0xe frozen Aug 16 16:12:20 blackhole kernel: ata6.00: irq_stat 0x08000000, interface fatal error Aug 16 16:12:20 blackhole kernel: ata6: SError: { Persist HostInt PHYRdyChg 10B8B Handshk } Aug 16 16:12:20 blackhole kernel: ata6.00: failed command: WRITE FPDMA QUEUED Aug 16 16:12:20 blackhole kernel: ata6.00: cmd 61/00:00:38:88:09/04:00:00:00:00/40 tag 0 ncq 524288 out#012 res 40/00:d8:38:f4:09/00:00:00:00:00/40 Emask 0x50 (ATA bus error) Aug 16 16:12:20 blackhole kernel: ata6.00: status: { DRDY } ... Aug 16 16:12:32 blackhole kernel: sd 5:0:0:0: [sdf] FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE Aug 16 16:12:32 blackhole kernel: sd 5:0:0:0: [sdf] Sense Key : Illegal Request [current] [descriptor] Aug 16 16:12:32 blackhole kernel: sd 5:0:0:0: [sdf] Add. Sense: Unaligned write command Aug 16 16:12:32 blackhole kernel: sd 5:0:0:0: [sdf] CDB: Write(10) 2a 00 00 09 88 38 00 04 00 00 Aug 16 16:12:32 blackhole kernel: blk_update_request: 23 callbacks suppressed Aug 16 16:12:32 blackhole kernel: blk_update_request: I/O error, dev sdf, sector 624696 Now, I have few questions: - is the above explanation plausible, or I am (horribly) missing something? - why the scsi midlevel does not respond to a power loss event by immediately offlining the disks? - is the scsi midlevel behavior configurable (I know I can lower eh timeout, but is this the right solution)? - how to deal with this problem (other than being 100% sure power is never lost by any disks)? Thank you all, regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 8:23 ` Gionatan Danti @ 2017-08-17 12:41 ` Roger Heflin 2017-08-17 14:31 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Roger Heflin @ 2017-08-17 12:41 UTC (permalink / raw) To: Gionatan Danti; +Cc: Reindl Harald, Roman Mamedov, Linux RAID On Thu, Aug 17, 2017 at 3:23 AM, Gionatan Danti <g.danti@assyoma.it> wrote: > On 14/07/2017 12:46, Gionatan Danti wrote:> Hi, so a premature/preventive > drive detachment is not a silver bullet, > but is this the right solution)? > - how to deal with this problem (other than being 100% sure power is never > lost by any disks)? > > Thank you all, > regards. > Here is a guess based on what you determined was the cause. The mid-layer does not know the writes were lost. The writes were in the drives write cache (already submitted to the drive and confirmed back to the mid-layer as done, even though they were not yet on the platter), and when the driver lost power and "rebooted" those writes disappeared, the write(s) the mid-layer had in progress and that never got a done from the drive failed were retried and succeeded after the driver reset was completed. In high reliability raid the solution is to turn off that write cache, *but* if you do direct io writes (most databases) with the drives write cache off and no battery backed up cache between the 2 then the drive becomes horribly slow since it must actually write the data to the platter before telling the next level up that the data was safe. ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 12:41 ` Roger Heflin @ 2017-08-17 14:31 ` Gionatan Danti 2017-08-17 17:33 ` Wols Lists 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-17 14:31 UTC (permalink / raw) To: Roger Heflin; +Cc: Reindl Harald, Roman Mamedov, Linux RAID Il 17-08-2017 14:41 Roger Heflin ha scritto: > > Here is a guess based on what you determined was the cause. > > The mid-layer does not know the writes were lost. The writes were in > the drives write cache (already submitted to the drive and confirmed > back to the mid-layer as done, even though they were not yet on the > platter), and when the driver lost power and "rebooted" those writes > disappeared, the write(s) the mid-layer had in progress and that never > got a done from the drive failed were retried and succeeded after the > driver reset was completed. > > In high reliability raid the solution is to turn off that write cache, > *but* if you do direct io writes (most databases) with the drives > write cache off and no battery backed up cache between the 2 then the > drive becomes horribly slow since it must actually write the data to > the platter before telling the next level up that the data was safe. Sure, disabling caching should at least greatly reduce the problem (torn writes remain a problem, but their are inevitable). However, the entire idea of barriers/cache flushes/FUAs was to *safely enable* unprotected write caches, even in the face of powerloss. Indeed, for full-system powerloss their are adequate. However, device-level micro-powerlosses seem to pose an bigger threat to data reliability. I suspect that the recurrent "my RAID1 array develops huge amount of mismatch_cnt sectors" question, which is often labeled as "don't worry about RAID1 mismatches", really has a strong tie with this specific problem. I suggest anyone reading this list to also read the current thread on the linux-scsi list - it is very interesting. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 14:31 ` Gionatan Danti @ 2017-08-17 17:33 ` Wols Lists 2017-08-17 20:50 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Wols Lists @ 2017-08-17 17:33 UTC (permalink / raw) To: Gionatan Danti, Roger Heflin; +Cc: Reindl Harald, Roman Mamedov, Linux RAID On 17/08/17 15:31, Gionatan Danti wrote: > However, the entire idea of barriers/cache flushes/FUAs was to *safely > enable* unprotected write caches, even in the face of powerloss. Indeed, > for full-system powerloss their are adequate. However, device-level > micro-powerlosses seem to pose an bigger threat to data reliability. Which is fine until the drive, bluntly put, lies to you. Cheaper drives are prone to this, in order to look good in benchmarks. Especially as it's hard to detect until you get screwed over by exactly this sort of thing. Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 17:33 ` Wols Lists @ 2017-08-17 20:50 ` Gionatan Danti 2017-08-17 21:01 ` Roger Heflin 2017-08-17 22:51 ` Wols Lists 0 siblings, 2 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-17 20:50 UTC (permalink / raw) To: Wols Lists; +Cc: Roger Heflin, Reindl Harald, Roman Mamedov, Linux RAID Il 17-08-2017 19:33 Wols Lists ha scritto: > Which is fine until the drive, bluntly put, lies to you. Cheaper drives > are prone to this, in order to look good in benchmarks. Especially as > it's hard to detect until you get screwed over by exactly this sort of > thing. It's more complex, actually. The hardware did not "lie" to me, as it correcly flushes caches when instructed to do. The problem is that a micro-powerloss wiped the cache *before* the drive had a chance to flush it, and the operating system did not detect this condition. From what I read on the linux-scsi and linux-ide lists, the host OS can not tell between a SATA link glitch and a SATA poweroff/poweron. This sound to me as a SATA specification problem, rather than a disk/OS one. However, a fix should be possible by examining some specific SMART values, which identify the powerloss/poweron condition. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 20:50 ` Gionatan Danti @ 2017-08-17 21:01 ` Roger Heflin 2017-08-17 21:21 ` Gionatan Danti 2017-08-17 22:51 ` Wols Lists 1 sibling, 1 reply; 46+ messages in thread From: Roger Heflin @ 2017-08-17 21:01 UTC (permalink / raw) To: Gionatan Danti; +Cc: Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID But even if you figured out which it was, you would have no way to know what writes were still sitting in the cache, it could be pretty much any writes from the last few seconds (or longer depending on how exactly the drive firmware works), and it would add additional complexity to keep a list of recent writes to validate actually happened in the case of an unexpected drive reset. This is probably more of a avoid this failure condition since this failure condition is not a normal failure mode and more of a very rare failure mode. On Thu, Aug 17, 2017 at 3:50 PM, Gionatan Danti <g.danti@assyoma.it> wrote: > Il 17-08-2017 19:33 Wols Lists ha scritto: >> >> Which is fine until the drive, bluntly put, lies to you. Cheaper drives >> are prone to this, in order to look good in benchmarks. Especially as >> it's hard to detect until you get screwed over by exactly this sort of >> thing. > > > It's more complex, actually. The hardware did not "lie" to me, as it > correcly flushes caches when instructed to do. > The problem is that a micro-powerloss wiped the cache *before* the drive had > a chance to flush it, and the operating system did not detect this > condition. > > From what I read on the linux-scsi and linux-ide lists, the host OS can not > tell between a SATA link glitch and a SATA poweroff/poweron. This sound to > me as a SATA specification problem, rather than a disk/OS one. However, a > fix should be possible by examining some specific SMART values, which > identify the powerloss/poweron condition. > > Regards. > > -- > Danti Gionatan > Supporto Tecnico > Assyoma S.r.l. - www.assyoma.it > email: g.danti@assyoma.it - info@assyoma.it > GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 21:01 ` Roger Heflin @ 2017-08-17 21:21 ` Gionatan Danti 2017-08-17 21:23 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-17 21:21 UTC (permalink / raw) To: Roger Heflin; +Cc: Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID Il 17-08-2017 23:01 Roger Heflin ha scritto: > But even if you figured out which it was, you would have no way to > know what writes were still sitting in the cache, it could be pretty > much any writes from the last few seconds (or longer depending on how > exactly the drive firmware works), and it would add additional > complexity to keep a list of recent writes to validate actually > happened in the case of an unexpected drive reset. This is probably > more of a avoid this failure condition since this failure condition is > not a normal failure mode and more of a very rare failure mode. Yes, but having identified the power-cycled disk, the system can not take the most sensible action. For example, it can re-sync it with its mirror disk, basically treating it as a --add-spare action. Or it can simply considering the disk as failing, kicking off it from the array and sending an alert email. What the system should not do is doing nothing: as differences accumulates, reading from the array become non-deterministic. In other words, two reads can produce two different results, based on what disk was queried. This *will* cause all sort of problems. Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 21:21 ` Gionatan Danti @ 2017-08-17 21:23 ` Gionatan Danti 0 siblings, 0 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-17 21:23 UTC (permalink / raw) To: Roger Heflin Cc: Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID, linux-raid-owner Il 17-08-2017 23:21 Gionatan Danti ha scritto: > Il 17-08-2017 23:01 Roger Heflin ha scritto: >> But even if you figured out which it was, you would have no way to >> know what writes were still sitting in the cache, it could be pretty >> much any writes from the last few seconds (or longer depending on how >> exactly the drive firmware works), and it would add additional >> complexity to keep a list of recent writes to validate actually >> happened in the case of an unexpected drive reset. This is probably >> more of a avoid this failure condition since this failure condition is >> not a normal failure mode and more of a very rare failure mode. > > Yes, but having identified the power-cycled disk, the system can not > take the most sensible action. Sorry, this should read: "Yes, but having identified the power-cycled disk, the system can *now* take the most sensible action" > For example, it can re-sync it with its mirror disk, basically > treating it as a --add-spare action. > Or it can simply considering the disk as failing, kicking off it from > the array and sending an alert email. > > What the system should not do is doing nothing: as differences > accumulates, reading from the array become non-deterministic. In other > words, two reads can produce two different results, based on what disk > was queried. This *will* cause all sort of problems. > > Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 20:50 ` Gionatan Danti 2017-08-17 21:01 ` Roger Heflin @ 2017-08-17 22:51 ` Wols Lists 2017-08-18 12:26 ` Gionatan Danti 1 sibling, 1 reply; 46+ messages in thread From: Wols Lists @ 2017-08-17 22:51 UTC (permalink / raw) To: Gionatan Danti; +Cc: Roger Heflin, Reindl Harald, Roman Mamedov, Linux RAID On 17/08/17 21:50, Gionatan Danti wrote: > > It's more complex, actually. The hardware did not "lie" to me, as it > correcly flushes caches when instructed to do. > The problem is that a micro-powerloss wiped the cache *before* the drive > had a chance to flush it, and the operating system did not detect this > condition. Except that that is not what should be happening. I don't know my hard drive details, but I believe drives have an instruction "async write this data and let me know when you have done so". This should NOT return "yes I've flushed it TO cache". Which is how you get your problem - the level above thinks it's been safely flushed to disk (because the disk has said "yes I've got it"), but it then gets lost because of your power fluctuation. It should only acknowledge it *after* it's been flushed *from* cache. And this is apparently exactly what cheap drives do ... If the level above says "tell me when it's safely on disk", and the drive truly does as its told, your problem won't happen because the disk block layer will time out waiting for the acknowledgement and retry the write. Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-17 22:51 ` Wols Lists @ 2017-08-18 12:26 ` Gionatan Danti 2017-08-18 12:54 ` Roger Heflin 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-18 12:26 UTC (permalink / raw) To: Wols Lists; +Cc: Roger Heflin, Reindl Harald, Roman Mamedov, Linux RAID Il 18-08-2017 00:51 Wols Lists ha scritto: > Except that that is not what should be happening. I don't know my hard > drive details, but I believe drives have an instruction "async write > this data and let me know when you have done so". > > This should NOT return "yes I've flushed it TO cache". Which is how you > get your problem - the level above thinks it's been safely flushed to > disk (because the disk has said "yes I've got it"), but it then gets > lost because of your power fluctuation. It should only acknowledge it > *after* it's been flushed *from* cache. > > And this is apparently exactly what cheap drives do ... > > If the level above says "tell me when it's safely on disk", and the > drive truly does as its told, your problem won't happen because the > disk > block layer will time out waiting for the acknowledgement and retry the > write. SATA drives generally guarantee persistent storage on physical medium by issuing *two* different FLUSH_CACHE commands, which do *not* form an atomic operation. In other words, it's not a problem of "cheap drives" or "lying hardware", rather, it seems a specific SATA limitation. This means the problem can not be solved by simply "buying better disks". Traditional flushing/barrier infrastructure simply has *no* method to ensure an atomic commit at the hardware level, and if something goes wrong between the two flushes, a (small) possibility exists to have corrupted writes without I/O errors reported to the upper layer, even in case of sync() writes. It's basically as a failing DRAM cache, but with *no* real failures... Newer drivers should implement FUAs, but I don't know if libata alredy uses them by default. Anyway, the disk's firmware is free to split a single FUA in more internal operations, so I am not sure they solves all problems. I really found the linux-scsi discussion interesting. Give it a look... Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-18 12:26 ` Gionatan Danti @ 2017-08-18 12:54 ` Roger Heflin 2017-08-18 19:42 ` Gionatan Danti 0 siblings, 1 reply; 46+ messages in thread From: Roger Heflin @ 2017-08-18 12:54 UTC (permalink / raw) To: Gionatan Danti; +Cc: Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID I have noticed all of the hardware raid controllers explicitly turn off the disk's write cache so this would eliminate this issue, but the cost is much slower write times. It makes the hardware raid controllers (and disk arrays) become uselessly slow when their battery backup dies and disables the raid card and/or arrays write cache. Remember, safe, fast and cheap, you only get to pick 2. We generally pick fast and cheap, the disk arrays/raid controllers pick safe and fast, but not so cheap as a hardware raid controller with write cache backup of some sort are quite expensive. On Fri, Aug 18, 2017 at 7:26 AM, Gionatan Danti <g.danti@assyoma.it> wrote: > Il 18-08-2017 00:51 Wols Lists ha scritto: >> >> Except that that is not what should be happening. I don't know my hard >> drive details, but I believe drives have an instruction "async write >> this data and let me know when you have done so". >> >> This should NOT return "yes I've flushed it TO cache". Which is how you >> get your problem - the level above thinks it's been safely flushed to >> disk (because the disk has said "yes I've got it"), but it then gets >> lost because of your power fluctuation. It should only acknowledge it >> *after* it's been flushed *from* cache. >> >> And this is apparently exactly what cheap drives do ... >> >> If the level above says "tell me when it's safely on disk", and the >> drive truly does as its told, your problem won't happen because the disk >> block layer will time out waiting for the acknowledgement and retry the >> write. > > > SATA drives generally guarantee persistent storage on physical medium by > issuing *two* different FLUSH_CACHE commands, which do *not* form an atomic > operation. In other words, it's not a problem of "cheap drives" or "lying > hardware", rather, it seems a specific SATA limitation. > > This means the problem can not be solved by simply "buying better disks". > Traditional flushing/barrier infrastructure simply has *no* method to ensure > an atomic commit at the hardware level, and if something goes wrong between > the two flushes, a (small) possibility exists to have corrupted writes > without I/O errors reported to the upper layer, even in case of sync() > writes. It's basically as a failing DRAM cache, but with *no* real > failures... > > Newer drivers should implement FUAs, but I don't know if libata alredy uses > them by default. Anyway, the disk's firmware is free to split a single FUA > in more internal operations, so I am not sure they solves all problems. > > I really found the linux-scsi discussion interesting. Give it a look... > > > Regards. > > -- > Danti Gionatan > Supporto Tecnico > Assyoma S.r.l. - www.assyoma.it > email: g.danti@assyoma.it - info@assyoma.it > GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-18 12:54 ` Roger Heflin @ 2017-08-18 19:42 ` Gionatan Danti 2017-08-20 7:14 ` Mikael Abrahamsson 0 siblings, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-18 19:42 UTC (permalink / raw) To: Roger Heflin; +Cc: Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID Il 18-08-2017 14:54 Roger Heflin ha scritto: > I have noticed all of the hardware raid controllers explicitly turn > off the disk's write cache so this would eliminate this issue, but the > cost is much slower write times. True... > It makes the hardware raid controllers (and disk arrays) become > uselessly slow when their battery backup dies and disables the raid > card and/or arrays write cache. ...true... > Remember, safe, fast and cheap, you only get to pick 2. We generally > pick fast and cheap, the disk arrays/raid controllers pick safe and > fast, but not so cheap as a hardware raid controller with write cache > backup of some sort are quite expensive. ...and true. I am not arguing any of these points. What really surprised me was to realize that, facing micro-powerlosses, *even sync() writes* can be vulnerable to undetected data loss, at least when not using FUAs (using instead the common barrier infrastructure). So while many (old) mismatch_cnt reports on RAID1/10 arrays where dismissed as "don't bother, it's a harmless RAID1 thing", I really think than some were genuine corruptions due to micro powerlosses and similar causes. If nothing more, such reports really emphasize the need to have a "trusted" mismatch_cnt for mirrored arrays, even in the face of some performance losses (due to no using zero copy anymore). Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-18 19:42 ` Gionatan Danti @ 2017-08-20 7:14 ` Mikael Abrahamsson 2017-08-20 7:24 ` Gionatan Danti 2017-08-20 23:22 ` Chris Murphy 0 siblings, 2 replies; 46+ messages in thread From: Mikael Abrahamsson @ 2017-08-20 7:14 UTC (permalink / raw) To: Gionatan Danti Cc: Roger Heflin, Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID On Fri, 18 Aug 2017, Gionatan Danti wrote: > So while many (old) mismatch_cnt reports on RAID1/10 arrays where > dismissed as "don't bother, it's a harmless RAID1 thing", I really think > than some were genuine corruptions due to micro powerlosses and similar > causes. After a non-clean poweroff and possible mismatch now between the RAID1 drives, and now fsck runs. It reads from the drives and fixes problem. However because the RAID1 drives contain different information, some of the errors are not fixed. Next time anything comes along, it might read from a different drive than what fsck read from, and now we have corruption. Wouldn't it make sense for an option where fsck can do its reads and the md layer would run "repair" on all stripes that fsck touches? Whatever information is handed off to fsck, then parity is always checked (and repaired) if there is a mismatch. The problem here with issuing a "repair" action is that it might actually copy data from the drive that fsck didn't read from, so now even though fsck thought it had made everything clean in the fs, it's no longer clean because md "repair" copied non-clean inforamation to the drive that fsck looked at and deemed to be ok? -- Mikael Abrahamsson email: swmike@swm.pp.se ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 7:14 ` Mikael Abrahamsson @ 2017-08-20 7:24 ` Gionatan Danti 2017-08-20 10:43 ` Mikael Abrahamsson 2017-08-20 23:22 ` Chris Murphy 1 sibling, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-20 7:24 UTC (permalink / raw) To: Mikael Abrahamsson Cc: Roger Heflin, Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID Il 20-08-2017 09:14 Mikael Abrahamsson ha scritto: > After a non-clean poweroff and possible mismatch now between the RAID1 > drives, and now fsck runs. It reads from the drives and fixes problem. > However because the RAID1 drives contain different information, some > of the errors are not fixed. Next time anything comes along, it might > read from a different drive than what fsck read from, and now we have > corruption. It can be even worse: if fsck reads from the disks with corrupted data and tries to repair based on these corrupted information, it can blow up the filesystem completely. In my case, heavy XFS corruption was prevented by the journal metadata checksum, which detected a corrupted journal and stopped mounting. However, some minor corruption found their ways onto the dentry/inode structures. Being a backup machine, this was not a big deal, as I simply recreated the filesystem from scratch. However, the failure mode (synced writes which were corrupted) was quite scary. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 7:24 ` Gionatan Danti @ 2017-08-20 10:43 ` Mikael Abrahamsson 2017-08-20 13:07 ` Wols Lists 2017-08-31 22:55 ` Robert L Mathews 0 siblings, 2 replies; 46+ messages in thread From: Mikael Abrahamsson @ 2017-08-20 10:43 UTC (permalink / raw) To: Gionatan Danti; +Cc: Linux RAID On Sun, 20 Aug 2017, Gionatan Danti wrote: > It can be even worse: if fsck reads from the disks with corrupted data > and tries to repair based on these corrupted information, it can blow up > the filesystem completely. Indeed, but as far as I know there is nothing md can do about this. What md could do about it is at least present a consistent view of data to fsck (which for raid1 would be read all stripes and issue "repair" if they don't match). Yes, this might indeed cause corruption but at least it would be consistent and visible. -- Mikael Abrahamsson email: swmike@swm.pp.se ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 10:43 ` Mikael Abrahamsson @ 2017-08-20 13:07 ` Wols Lists 2017-08-20 15:38 ` Adam Goryachev 2017-08-20 19:01 ` Gionatan Danti 2017-08-31 22:55 ` Robert L Mathews 1 sibling, 2 replies; 46+ messages in thread From: Wols Lists @ 2017-08-20 13:07 UTC (permalink / raw) To: Mikael Abrahamsson, Gionatan Danti; +Cc: Linux RAID On 20/08/17 11:43, Mikael Abrahamsson wrote: > On Sun, 20 Aug 2017, Gionatan Danti wrote: > >> It can be even worse: if fsck reads from the disks with corrupted data >> and tries to repair based on these corrupted information, it can blow >> up the filesystem completely. > > Indeed, but as far as I know there is nothing md can do about this. What > md could do about it is at least present a consistent view of data to > fsck (which for raid1 would be read all stripes and issue "repair" if > they don't match). Yes, this might indeed cause corruption but at least > it would be consistent and visible. > Which is exactly what my "force integrity check on read" proposal would have achieved, but that generated so much heat and argument IN FAVOUR of returning possibly corrupt data that I'll probably get flamed to high heaven if I bring it back up again. Yes, the performance hit is probably awful, yes it can only fix things if it's got raid-6 or a 3-disk-or-more raid-1 array, but the idea was that if you knew or suspected something was wrong, this would force a read error somewhere in the stack if the raid wasn't consistent. Switching it on then running your fsck might trash chunks of the filesystem, but at least (a) it would be known to be consistent afterwards, and (b) you'd know what had been trashed! Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 13:07 ` Wols Lists @ 2017-08-20 15:38 ` Adam Goryachev 2017-08-20 15:48 ` Mikael Abrahamsson 2017-08-20 19:03 ` Gionatan Danti 2017-08-20 19:01 ` Gionatan Danti 1 sibling, 2 replies; 46+ messages in thread From: Adam Goryachev @ 2017-08-20 15:38 UTC (permalink / raw) To: Wols Lists, Mikael Abrahamsson, Gionatan Danti; +Cc: Linux RAID On 20/8/17 23:07, Wols Lists wrote: > On 20/08/17 11:43, Mikael Abrahamsson wrote: >> On Sun, 20 Aug 2017, Gionatan Danti wrote: >> >>> It can be even worse: if fsck reads from the disks with corrupted data >>> and tries to repair based on these corrupted information, it can blow >>> up the filesystem completely. >> Indeed, but as far as I know there is nothing md can do about this. What >> md could do about it is at least present a consistent view of data to >> fsck (which for raid1 would be read all stripes and issue "repair" if >> they don't match). Yes, this might indeed cause corruption but at least >> it would be consistent and visible. >> > Which is exactly what my "force integrity check on read" proposal would > have achieved, but that generated so much heat and argument IN FAVOUR of > returning possibly corrupt data that I'll probably get flamed to high > heaven if I bring it back up again. Yes, the performance hit is probably > awful, yes it can only fix things if it's got raid-6 or a 3-disk-or-more > raid-1 array, but the idea was that if you knew or suspected something > was wrong, this would force a read error somewhere in the stack if the > raid wasn't consistent. > > Switching it on then running your fsck might trash chunks of the > filesystem, but at least (a) it would be known to be consistent > afterwards, and (b) you'd know what had been trashed! In the case where you know there are "probably" some inconsistencies, you have a few choices: 1) If you know which disk is faulty, then fail it, then clean the superblock and add it. It will be re-written from the known good drive 2) If you don't know which drive is faulty, or both drives accrued random write errors, then all you can do is make sure that both drives have the same data (even where it is wrong). So just do a check/repair which will ensure both drives are consistent, then you can safely do the fsck. (Assuming you fixed the problem causing random write errors first). Your proposed option to read from all (or at least 2) data sources to ensure data consistency is an online version of the above process in (2), not a bad tool to have available, but not required in this scenario (IMHO). It is more useful when you think all drives are OK, and you want to be *sure* that they are OK on a continuous basis, not just after you think there might be a problem. While I suspect patches would be accepted, without someone capable of actually writing the code being interested, then it probably won't happen (until one of those people needs it). Regards, Adam ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 15:38 ` Adam Goryachev @ 2017-08-20 15:48 ` Mikael Abrahamsson 2017-08-20 16:10 ` Wols Lists 2017-08-20 19:11 ` Gionatan Danti 2017-08-20 19:03 ` Gionatan Danti 1 sibling, 2 replies; 46+ messages in thread From: Mikael Abrahamsson @ 2017-08-20 15:48 UTC (permalink / raw) To: Adam Goryachev; +Cc: Linux RAID On Mon, 21 Aug 2017, Adam Goryachev wrote: > data (even where it is wrong). So just do a check/repair which will > ensure both drives are consistent, then you can safely do the fsck. > (Assuming you fixed the problem causing random write errors first). This involves manual intervention. While I don't know how to implement this, let's at least see if we can architect something for throwing ideas around. What about having an option for any raid level that would do "repair on read". So you can do "0" or "1" on this. RAID1 would mean it reads all stripes and if there is inconsistency, pick one and write it to all of them. It could also be some kind of IOCTL option I guess. For RAID5/6, read all data drives, and check parity. If parity is wrong, write parity. This could mean that if filesystem developers wanted to do repair (and this could be a userspace option or mount option), it would use the beforementioned option for all fsck-like operation to make sure that metadata was consistent while doing fsck (this would be different for different tools, if it's an "fs needs to be mounted"-type of fs, or if it's an "offline fsck" type filesystem. Then it could go back to normal operation for everything else that would hopefully not cause catastrophical failures to the filesystem, but instead just individual file corruption in case of mismatches. -- Mikael Abrahamsson email: swmike@swm.pp.se ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 15:48 ` Mikael Abrahamsson @ 2017-08-20 16:10 ` Wols Lists 2017-08-20 23:11 ` Adam Goryachev 2017-08-20 19:11 ` Gionatan Danti 1 sibling, 1 reply; 46+ messages in thread From: Wols Lists @ 2017-08-20 16:10 UTC (permalink / raw) To: Mikael Abrahamsson, Adam Goryachev; +Cc: Linux RAID On 20/08/17 16:48, Mikael Abrahamsson wrote: > On Mon, 21 Aug 2017, Adam Goryachev wrote: > >> data (even where it is wrong). So just do a check/repair which will >> ensure both drives are consistent, then you can safely do the fsck. >> (Assuming you fixed the problem causing random write errors first). > > This involves manual intervention. > > While I don't know how to implement this, let's at least see if we can > architect something for throwing ideas around. > > What about having an option for any raid level that would do "repair on > read". So you can do "0" or "1" on this. RAID1 would mean it reads all > stripes and if there is inconsistency, pick one and write it to all of > them. It could also be some kind of IOCTL option I guess. For RAID5/6, > read all data drives, and check parity. If parity is wrong, write parity. > > This could mean that if filesystem developers wanted to do repair (and > this could be a userspace option or mount option), it would use the > beforementioned option for all fsck-like operation to make sure that > metadata was consistent while doing fsck (this would be different for > different tools, if it's an "fs needs to be mounted"-type of fs, or if > it's an "offline fsck" type filesystem. Then it could go back to normal > operation for everything else that would hopefully not cause > catastrophical failures to the filesystem, but instead just individual > file corruption in case of mismatches. > Look for the thread "RFC Raid error detection and auto-recovery, 10th May. Basically, that proposed a three-way flag - "default" is the current "read the data section", "check" would read the entire stripe and compare a mirror or calculate parity on a raid and return a read error if it couldn't work out the correct data, and "fix" would write the correct data back if it could work it out. So basically, on a two-disk raid-1, or raid 4 or 5, both "check" and "fix" would return read errors if there's a problem and you're SOL without a backup. With a three-disk or more raid-1, or raid-6, it would return the correct data (and fix the stripe) if it could, otherwise again you're SOL. Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 16:10 ` Wols Lists @ 2017-08-20 23:11 ` Adam Goryachev 2017-08-21 14:03 ` Anthony Youngman 0 siblings, 1 reply; 46+ messages in thread From: Adam Goryachev @ 2017-08-20 23:11 UTC (permalink / raw) To: Wols Lists, Mikael Abrahamsson; +Cc: Linux RAID On 21/08/17 02:10, Wols Lists wrote: > On 20/08/17 16:48, Mikael Abrahamsson wrote: >> On Mon, 21 Aug 2017, Adam Goryachev wrote: >> >>> data (even where it is wrong). So just do a check/repair which will >>> ensure both drives are consistent, then you can safely do the fsck. >>> (Assuming you fixed the problem causing random write errors first). >> This involves manual intervention. >> >> While I don't know how to implement this, let's at least see if we can >> architect something for throwing ideas around. >> >> What about having an option for any raid level that would do "repair on >> read". So you can do "0" or "1" on this. RAID1 would mean it reads all >> stripes and if there is inconsistency, pick one and write it to all of >> them. It could also be some kind of IOCTL option I guess. For RAID5/6, >> read all data drives, and check parity. If parity is wrong, write parity. >> >> This could mean that if filesystem developers wanted to do repair (and >> this could be a userspace option or mount option), it would use the >> beforementioned option for all fsck-like operation to make sure that >> metadata was consistent while doing fsck (this would be different for >> different tools, if it's an "fs needs to be mounted"-type of fs, or if >> it's an "offline fsck" type filesystem. Then it could go back to normal >> operation for everything else that would hopefully not cause >> catastrophical failures to the filesystem, but instead just individual >> file corruption in case of mismatches. >> > Look for the thread "RFC Raid error detection and auto-recovery, 10th May. > > Basically, that proposed a three-way flag - "default" is the current > "read the data section", "check" would read the entire stripe and > compare a mirror or calculate parity on a raid and return a read error > if it couldn't work out the correct data, and "fix" would write the > correct data back if it could work it out. > > So basically, on a two-disk raid-1, or raid 4 or 5, both "check" and > "fix" would return read errors if there's a problem and you're SOL > without a backup. > > With a three-disk or more raid-1, or raid-6, it would return the correct > data (and fix the stripe) if it could, otherwise again you're SOL. From memory, the main sticking point was in implementing this with RAID6 and the argument that you might not be able to choose the "right" pieces of data because there wasn't a sufficient amount of data to know which was corrupted. Perhaps it would be a easier starting point to use RAID1 with a three (or more) mirrors to implement this. You only need to read two drives to "check" that there is consensus (technically, int(n/2)+1, though you could start with just 2 which ensures there isn't one drive behaving badly). Once this is implemented, if you need larger arrays, then you would need to layer your RAID, using RAID61 with >=3 mirror RAID1 components. Eventually, you might be able to migrate this to RAID6 or other levels, but at least once it is in kernel, and proven to be working (and actually used by people) then it will get a lot easier. Regards, Adam -- Adam Goryachev Website Managers www.websitemanagers.com.au -- The information in this e-mail is confidential and may be legally privileged. It is intended solely for the addressee. Access to this e-mail by anyone else is unauthorised. If you are not the intended recipient, any disclosure, copying, distribution or any action taken or omitted to be taken in reliance on it, is prohibited and may be unlawful. If you have received this message in error, please notify us immediately. Please also destroy and delete the message from your computer. Viruses - Any loss/damage incurred by receiving this email is not the sender's responsibility. ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 23:11 ` Adam Goryachev @ 2017-08-21 14:03 ` Anthony Youngman 0 siblings, 0 replies; 46+ messages in thread From: Anthony Youngman @ 2017-08-21 14:03 UTC (permalink / raw) To: Adam Goryachev, Mikael Abrahamsson; +Cc: Linux RAID On 21/08/17 00:11, Adam Goryachev wrote: > On 21/08/17 02:10, Wols Lists wrote: >> On 20/08/17 16:48, Mikael Abrahamsson wrote: >>> On Mon, 21 Aug 2017, Adam Goryachev wrote: >>> >>>> data (even where it is wrong). So just do a check/repair which will >>>> ensure both drives are consistent, then you can safely do the fsck. >>>> (Assuming you fixed the problem causing random write errors first). >>> This involves manual intervention. >>> >>> While I don't know how to implement this, let's at least see if we can >>> architect something for throwing ideas around. >>> >>> What about having an option for any raid level that would do "repair on >>> read". So you can do "0" or "1" on this. RAID1 would mean it reads all >>> stripes and if there is inconsistency, pick one and write it to all of >>> them. It could also be some kind of IOCTL option I guess. For RAID5/6, >>> read all data drives, and check parity. If parity is wrong, write >>> parity. >>> >>> This could mean that if filesystem developers wanted to do repair (and >>> this could be a userspace option or mount option), it would use the >>> beforementioned option for all fsck-like operation to make sure that >>> metadata was consistent while doing fsck (this would be different for >>> different tools, if it's an "fs needs to be mounted"-type of fs, or if >>> it's an "offline fsck" type filesystem. Then it could go back to normal >>> operation for everything else that would hopefully not cause >>> catastrophical failures to the filesystem, but instead just individual >>> file corruption in case of mismatches. >>> >> Look for the thread "RFC Raid error detection and auto-recovery, 10th >> May. >> >> Basically, that proposed a three-way flag - "default" is the current >> "read the data section", "check" would read the entire stripe and >> compare a mirror or calculate parity on a raid and return a read error >> if it couldn't work out the correct data, and "fix" would write the >> correct data back if it could work it out. >> >> So basically, on a two-disk raid-1, or raid 4 or 5, both "check" and >> "fix" would return read errors if there's a problem and you're SOL >> without a backup. >> >> With a three-disk or more raid-1, or raid-6, it would return the correct >> data (and fix the stripe) if it could, otherwise again you're SOL. > > From memory, the main sticking point was in implementing this with > RAID6 and the argument that you might not be able to choose the "right" > pieces of data because there wasn't a sufficient amount of data to know > which was corrupted. That was the impression I got, but I really don't understand the problem. If *ANY* one stripe is corrupted, we have two unknowns, two parity blocks, and we can recalculate the missing stripe. If two or more stripes are corrupt, the recovery will return garbage (which is detectable) and we return a read error. We DO NOT attempt to rewrite the stripe! In your words, if we can't choose the "right" piece of data, we bail and do nothing. As I understood it, the worry was that we would run the recovery algorithm and then overwrite the data with garbage, but nobody ever gave me a plausible scenario where that could happen. The only plausible scenario is where multiple stripes are corrupted in such a way that the recovery algorithm is fooled into thinking only one stripe is affected. And if I read that paper correctly, the odds of that happening are very low. Short summary - if just one stripe is corrupted, then my proposal will fix and return CORRECT data. If however, more than one stripe is corrupted, then my proposal will with near-perfect accuracy bail and do nothing (apart from returning a read error). As I say, the only risk to the data is if the error looks like a single-stripe problem when it isn't, and that's unlikely. I've had enough data-loss scenarios in my career to be rather paranoid about scribbling over stuff when I don't know what I'm doing ... (I do understand concerns about "using the wrong tool to fix the wrong problem", but you don't refuse to sell a punter a wheel-wrench because he might not be able to tell the difference between a flat tyre and a mis-firing engine). Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 15:48 ` Mikael Abrahamsson 2017-08-20 16:10 ` Wols Lists @ 2017-08-20 19:11 ` Gionatan Danti 1 sibling, 0 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-20 19:11 UTC (permalink / raw) To: Mikael Abrahamsson; +Cc: Adam Goryachev, Linux RAID, linux-raid-owner Il 20-08-2017 17:48 Mikael Abrahamsson ha scritto: > > This involves manual intervention. > > While I don't know how to implement this, let's at least see if we can > architect something for throwing ideas around. > > What about having an option for any raid level that would do "repair > on read". So you can do "0" or "1" on this. RAID1 would mean it reads > all stripes and if there is inconsistency, pick one and write it to > all of them. It could also be some kind of IOCTL option I guess. For > RAID5/6, read all data drives, and check parity. If parity is wrong, > write parity. Wait, is isn't that what MDRAID already do by issuing "echo 1 > sync_action"? The big plus would be to not blindly copy the first mirror/stripe, rather to identify the correct one and use it to correct any corrupted data. Obviously you need sufficient data to do that, by the mean of 3-way RAID1, double parity (RAID6) or checksummed data blocks (ZFS, BTRFS and dm-integrity). Please note that these methods alone do not provide complete protection over other failures mode as phantom writes; however, any of them would significantly increase the current data protection level. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 15:38 ` Adam Goryachev 2017-08-20 15:48 ` Mikael Abrahamsson @ 2017-08-20 19:03 ` Gionatan Danti 1 sibling, 0 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-20 19:03 UTC (permalink / raw) To: Adam Goryachev; +Cc: Wols Lists, Mikael Abrahamsson, Linux RAID Il 20-08-2017 17:38 Adam Goryachev ha scritto: > In the case where you know there are "probably" some inconsistencies, > you have a few choices: > 1) If you know which disk is faulty, then fail it, then clean the > superblock and add it. It will be re-written from the known good drive No need to clear the superblock. You can re-add it using the "--add-spare" option which will force a full re-sync from the mirror disk. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 13:07 ` Wols Lists 2017-08-20 15:38 ` Adam Goryachev @ 2017-08-20 19:01 ` Gionatan Danti 1 sibling, 0 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-20 19:01 UTC (permalink / raw) To: Wols Lists; +Cc: Mikael Abrahamsson, Linux RAID Il 20-08-2017 15:07 Wols Lists ha scritto: > Which is exactly what my "force integrity check on read" proposal would > have achieved, but that generated so much heat and argument IN FAVOUR > of > returning possibly corrupt data that I'll probably get flamed to high > heaven if I bring it back up again. I think the aversion to such an approach is due to: a) a *big* performance degradation (you get the IOPs of a single disk); b) the existence on all-encompassing checksummed filesystems as ZFS and BTRFS[1]; c) the difficulty to actually write such a code; d) a understimating of how often can these data-corruption problem happens in real life. I can not really blame MDRAID for what it provides, as it is incredibly flexible and very fast. Sure, a user-selectable option to auto discover/correct corrupted data would be great, but it seems that this is not the road MDRAID will ever take. However, a possible solution would be to use dm-integrity on top of the single component devices of an MDRAID array. Give at look at stratis[2], it will be interesting... [1] In the current state, I do not really trust BTRFS. I put much more hopes on ZoL... [2] https://stratis-storage.github.io/StratisSoftwareDesign.pdf -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 10:43 ` Mikael Abrahamsson 2017-08-20 13:07 ` Wols Lists @ 2017-08-31 22:55 ` Robert L Mathews 2017-09-01 5:39 ` Reindl Harald 1 sibling, 1 reply; 46+ messages in thread From: Robert L Mathews @ 2017-08-31 22:55 UTC (permalink / raw) To: Linux RAID On 8/20/17 3:43 AM, Mikael Abrahamsson wrote: > Indeed, but as far as I know there is nothing md can do about this. What > md could do about it is at least present a consistent view of data to > fsck (which for raid1 would be read all stripes and issue "repair" if > they don't match). Yes, this might indeed cause corruption but at least > it would be consistent and visible. If you set all disks except one as "write-mostly", won't mdadm give you a consistent view of it because it only reads from a single disk? (Sorry for the delayed reply to a two-week-old thread; I was on vacation.) -- Robert L Mathews, Tiger Technologies, http://www.tigertech.net/ ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-31 22:55 ` Robert L Mathews @ 2017-09-01 5:39 ` Reindl Harald 2017-09-01 23:14 ` Robert L Mathews 0 siblings, 1 reply; 46+ messages in thread From: Reindl Harald @ 2017-09-01 5:39 UTC (permalink / raw) To: Linux RAID Am 01.09.2017 um 00:55 schrieb Robert L Mathews: > On 8/20/17 3:43 AM, Mikael Abrahamsson wrote: > >> Indeed, but as far as I know there is nothing md can do about this. What >> md could do about it is at least present a consistent view of data to >> fsck (which for raid1 would be read all stripes and issue "repair" if >> they don't match). Yes, this might indeed cause corruption but at least >> it would be consistent and visible. > > If you set all disks except one as "write-mostly", won't mdadm give you > a consistent view of it because it only reads from a single disk? you gain nothing when you completly lie to fsck and after that switch back to normal operations with the other disks part of the game and it works only on RAID1 and *really* RAID1, otherwise my current RAID10 would be as fast as hell instead the terrible random lags which are worser sometimes as before replace two out of 4 disks with a SSD ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-09-01 5:39 ` Reindl Harald @ 2017-09-01 23:14 ` Robert L Mathews 0 siblings, 0 replies; 46+ messages in thread From: Robert L Mathews @ 2017-09-01 23:14 UTC (permalink / raw) To: Linux RAID On 8/31/17 10:39 PM, Reindl Harald wrote: > you gain nothing when you completly lie to fsck and after that switch > back to normal operations with the other disks part of the game Sorry, I didn't make myself clear. I meant that you could leave the array like that (all disks except one write-mostly) permanently, in normal use (and while fscking too). That way you always have a consistent view of the array. With an array composed of modern SSDs, such a setup still performs well for many loads. -- Robert L Mathews, Tiger Technologies, http://www.tigertech.net/ ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 7:14 ` Mikael Abrahamsson 2017-08-20 7:24 ` Gionatan Danti @ 2017-08-20 23:22 ` Chris Murphy 2017-08-21 5:57 ` Gionatan Danti 2017-08-21 8:37 ` Mikael Abrahamsson 1 sibling, 2 replies; 46+ messages in thread From: Chris Murphy @ 2017-08-20 23:22 UTC (permalink / raw) To: Mikael Abrahamsson Cc: Gionatan Danti, Roger Heflin, Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID On Sun, Aug 20, 2017 at 1:14 AM, Mikael Abrahamsson <swmike@swm.pp.se> wrote: > After a non-clean poweroff and possible mismatch now between the RAID1 > drives, and now fsck runs. It reads from the drives and fixes problem. > However because the RAID1 drives contain different information, some of the > errors are not fixed. Next time anything comes along, it might read from a > different drive than what fsck read from, and now we have corruption. The fsck has no idea this is two drives, it things it's one and does an overwrite of whatever (virtual) blocks contain file system metadata needing repair. Then md should take each fsck write, and duplicate it (for 2 way mirror) and push those writes to each real physical device. Since md doesn't read from both mirrors, it's possible there's a read from a non-corrupt drive, which presents good information to fsck, which then sees no reason to fix anything in that block; but the other mirror does have corruption which thus goes undetected. One way of dealing with it is to scrub (repair) so they both have the same information to hand over to fsck. Fixups then get replicated to disks by md. Another way is to split the mirror (make one device faulty), and then fix the remaining drive (now degraded). If that goes well, the 2nd device can be re-added. Here's a caveat thought: how it resync's will depend on the write-intent bitmap being present. I have no idea if write-intent bitmaps on two drives can get out of sync and what the ensuing behavior is, but I'd like to think md will discover the fixed drive event count is higher than the re-added one, and if necessary does a full resync, rather than possibly re-introducing any corruption. -- Chris Murphy ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 23:22 ` Chris Murphy @ 2017-08-21 5:57 ` Gionatan Danti 2017-08-21 8:37 ` Mikael Abrahamsson 1 sibling, 0 replies; 46+ messages in thread From: Gionatan Danti @ 2017-08-21 5:57 UTC (permalink / raw) To: Chris Murphy Cc: Mikael Abrahamsson, Roger Heflin, Wols Lists, Reindl Harald, Roman Mamedov, Linux RAID, chris Il 21-08-2017 01:22 Chris Murphy ha scritto: > Another way is to split the mirror (make one device faulty), and then > fix the remaining drive (now degraded). If that goes well, the 2nd > device can be re-added. Here's a caveat thought: how it resync's will > depend on the write-intent bitmap being present. I have no idea if > write-intent bitmaps on two drives can get out of sync and what the > ensuing behavior is, but I'd like to think md will discover the fixed > drive event count is higher than the re-added one, and if necessary > does a full resync, rather than possibly re-introducing any > corruption. On the corruption I am replicating (brief SATA power interruptions), the event count of both drives where identical, and so was the write bitmap (not always, thought). To be 100% sure to completly copy from the mirror device, you had to re-add the corrupted driver as a spare, by using "--add-spare". From the man page: "--add-spare Add a device as a spare. This is similar to --add except that it does not attempt --re-add first. The device will be added as a spare even if it looks like it could be an recent member of the array." Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-20 23:22 ` Chris Murphy 2017-08-21 5:57 ` Gionatan Danti @ 2017-08-21 8:37 ` Mikael Abrahamsson 2017-08-21 12:28 ` Gionatan Danti 2017-08-21 17:33 ` Chris Murphy 1 sibling, 2 replies; 46+ messages in thread From: Mikael Abrahamsson @ 2017-08-21 8:37 UTC (permalink / raw) To: Chris Murphy; +Cc: Linux RAID On Sun, 20 Aug 2017, Chris Murphy wrote: > Since md doesn't read from both mirrors, it's possible there's a read > from a non-corrupt drive, which presents good information to fsck, which > then sees no reason to fix anything in that block; but the other mirror > does have corruption which thus goes undetected. That was exactly what I wrote. > One way of dealing with it is to scrub (repair) so they both have the > same information to hand over to fsck. Fixups then get replicated to > disks by md. Yes, it is, but that would require a full repair before doing fsck. That seems excessive because that will take hours on larger drives. > Another way is to split the mirror (make one device faulty), and then > fix the remaining drive (now degraded). If that goes well, the 2nd > device can be re-added. Here's a caveat thought: how it resync's will > depend on the write-intent bitmap being present. I have no idea if > write-intent bitmaps on two drives can get out of sync and what the > ensuing behavior is, but I'd like to think md will discover the fixed > drive event count is higher than the re-added one, and if necessary > does a full resync, rather than possibly re-introducing any > corruption. This doesn't solve the problem because it doesn't check if the second mirror is out of sync with the first one, because it'll only detect writes to the degraded array and sync those. It doesn't fix the "fsck read the block and it was fine, but on the second drive it's not fine". In that case fsck would have to be modified to write all blocks it read to make them dirty, so they're sync:ed. However, this again causes the problem that if there is an URE on the degraded array remaining drive, things will fail. The only way to solve this is to add more code to implement a new mode which would be "repair-on-read". I understand that we can't necessarily detect which drive has the right or wrong information, but at least we can this way make sure that when fsck is done, all the inodes and other metadata is now consistent. Everything that fsck touched during the fsck will be consistent across all drives, with correct parity. It might not contain the "best" information that could have been presented by a more intelligent algorithm/metadata, but at least it's better than today when after a fsck run you don't know if parity is correct or not. It would also be a good diagnostic tool for admins. If you suspect that you're getting inconsistencies but you're fine with the performance degradation then md could log inconsistencies somewhere so you know about them. -- Mikael Abrahamsson email: swmike@swm.pp.se ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-21 8:37 ` Mikael Abrahamsson @ 2017-08-21 12:28 ` Gionatan Danti 2017-08-21 14:09 ` Anthony Youngman 2017-08-21 17:33 ` Chris Murphy 1 sibling, 1 reply; 46+ messages in thread From: Gionatan Danti @ 2017-08-21 12:28 UTC (permalink / raw) To: Mikael Abrahamsson; +Cc: Chris Murphy, Linux RAID, linux-raid-owner Il 21-08-2017 10:37 Mikael Abrahamsson ha scritto: > This doesn't solve the problem because it doesn't check if the second > mirror is out of sync with the first one, because it'll only detect > writes to the degraded array and sync those. It doesn't fix the "fsck > read the block and it was fine, but on the second drive it's not > fine". As stated elsewhere, you can re-attach a detached device with "--add-spare": this will copy *all* data from the other mirror leg. However, it is vastly better to simple issue a "repair" action. Anyway, the basic problem remains: with larger drives, this will take many hours or even days. > However, this again causes the problem that if there is an URE on the > degraded array remaining drive, things will fail. On relatively recent MDRAID code (kernel > 3.5.x), a degraded array with a URE in another disk will *not* totally fail the array. Rather, a badblock is logged into MDRAID superblock and a read error is returned to upper layers. Anyway, this has little to do with the main problem: micro power losses can cause undetected, silent data corruption, even with synced writes. > The only way to solve this is to add more code to implement a new mode > which would be "repair-on-read". > > I understand that we can't necessarily detect which drive has the > right or wrong information, but at least we can this way make sure > that when fsck is done, all the inodes and other metadata is now > consistent. Everything that fsck touched during the fsck will be > consistent across all drives, with correct parity. It might not > contain the "best" information that could have been presented by a > more intelligent algorithm/metadata, but at least it's better than > today when after a fsck run you don't know if parity is correct or > not. > > It would also be a good diagnostic tool for admins. If you suspect > that you're getting inconsistencies but you're fine with the > performance degradation then md could log inconsistencies somewhere so > you know about them. I second that. Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-21 12:28 ` Gionatan Danti @ 2017-08-21 14:09 ` Anthony Youngman 0 siblings, 0 replies; 46+ messages in thread From: Anthony Youngman @ 2017-08-21 14:09 UTC (permalink / raw) To: Gionatan Danti, Mikael Abrahamsson Cc: Chris Murphy, Linux RAID, linux-raid-owner On 21/08/17 13:28, Gionatan Danti wrote: >> It would also be a good diagnostic tool for admins. If you suspect >> that you're getting inconsistencies but you're fine with the >> performance degradation then md could log inconsistencies somewhere so >> you know about them. > > I second that. > Thanks. Sounds like I should try and write the code for my RFC then :-) Just be prepared for a lot of requests for help ... :-) Another item on my long list of "I'll do it when I get the chance" things :-( Cheers, Wol ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-21 8:37 ` Mikael Abrahamsson 2017-08-21 12:28 ` Gionatan Danti @ 2017-08-21 17:33 ` Chris Murphy 2017-08-21 17:52 ` Reindl Harald 1 sibling, 1 reply; 46+ messages in thread From: Chris Murphy @ 2017-08-21 17:33 UTC (permalink / raw) To: Mikael Abrahamsson; +Cc: Chris Murphy, Linux RAID On Mon, Aug 21, 2017 at 2:37 AM, Mikael Abrahamsson <swmike@swm.pp.se> wrote: > On Sun, 20 Aug 2017, Chris Murphy wrote: > >> Since md doesn't read from both mirrors, it's possible there's a read from >> a non-corrupt drive, which presents good information to fsck, which then >> sees no reason to fix anything in that block; but the other mirror does have >> corruption which thus goes undetected. > > > That was exactly what I wrote. > >> One way of dealing with it is to scrub (repair) so they both have the same >> information to hand over to fsck. Fixups then get replicated to disks by md. > > > Yes, it is, but that would require a full repair before doing fsck. That > seems excessive because that will take hours on larger drives. Hence we have ZFS and Btrfs and dm-integrity to unambiguously identify corruption and prevent it from escaping to higher levels. That you have multiple incongruencies with fs metadata, there's a good chance some data is also affected. Data is a much bigger percentage. Might as well bite the bullet and scrub the whole thing. > >> Another way is to split the mirror (make one device faulty), and then >> fix the remaining drive (now degraded). If that goes well, the 2nd >> device can be re-added. Here's a caveat thought: how it resync's will >> depend on the write-intent bitmap being present. I have no idea if >> write-intent bitmaps on two drives can get out of sync and what the >> ensuing behavior is, but I'd like to think md will discover the fixed >> drive event count is higher than the re-added one, and if necessary >> does a full resync, rather than possibly re-introducing any >> corruption. > > > This doesn't solve the problem because it doesn't check if the second mirror > is out of sync with the first one, because it'll only detect writes to the > degraded array and sync those. It doesn't fix the "fsck read the block and > it was fine, but on the second drive it's not fine". > > In that case fsck would have to be modified to write all blocks it read to > make them dirty, so they're sync:ed. OK so you have a corrupt underlying storage stack for possibly unknown reasons, and you're just going to take a chance and overwrite the entire file system. Seems like a bad hack to me, but I'd love to know what the ext4 and XFS devs think about it. The rule has always been get lower levels healthy first. Two mirrors that have the same even count but are not block identical is a broken array. -- Chris Murphy ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-08-21 17:33 ` Chris Murphy @ 2017-08-21 17:52 ` Reindl Harald 0 siblings, 0 replies; 46+ messages in thread From: Reindl Harald @ 2017-08-21 17:52 UTC (permalink / raw) To: Chris Murphy, Mikael Abrahamsson; +Cc: Linux RAID Am 21.08.2017 um 19:33 schrieb Chris Murphy: > On Mon, Aug 21, 2017 at 2:37 AM, Mikael Abrahamsson <swmike@swm.pp.se> wrote: >> On Sun, 20 Aug 2017, Chris Murphy wrote: >> >>> Since md doesn't read from both mirrors, it's possible there's a read from >>> a non-corrupt drive, which presents good information to fsck, which then >>> sees no reason to fix anything in that block; but the other mirror does have >>> corruption which thus goes undetected. >> >> >> That was exactly what I wrote. >> >>> One way of dealing with it is to scrub (repair) so they both have the same >>> information to hand over to fsck. Fixups then get replicated to disks by md. >> >> >> Yes, it is, but that would require a full repair before doing fsck. That >> seems excessive because that will take hours on larger drives. > > Hence we have ZFS and Btrfs and dm-integrity to unambiguously identify > corruption and prevent it from escaping to higher levels where do we have ZFS? where do we have *stable* BTRFS after Redhat gave up recently? ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-13 22:34 ` Gionatan Danti 2017-07-14 0:32 ` Reindl Harald @ 2017-07-14 1:48 ` Chris Murphy 2017-07-14 7:22 ` Roman Mamedov 1 sibling, 1 reply; 46+ messages in thread From: Chris Murphy @ 2017-07-14 1:48 UTC (permalink / raw) To: Linux-RAID On Thu, Jul 13, 2017 at 4:34 PM, Gionatan Danti <g.danti@assyoma.it> wrote: > Il 13-07-2017 23:34 Reindl Harald ha scritto: >> >> maybe because the disk is, well, not in a good shape and don't know >> that by itself >> > > But the kernel *does* know that, as the dmesg entries clearly show. > Basically, some SATA commands timed-out and/or were aborted. As the kernel > reported these erros in dmesg, why do not use these information to stop a > failing disk? > >> >> (and no filesystems with checksums won't magically recover >> your data, they just tell you realier they are gone) >> > > Checksummed filesystem that integrates their block-level management (read: > ZFS or BTRFS) can recover the missing/corrupted data by the healthy disks, > discarging corrupted data based on the checksum mismatch. > > Anyway, this has nothing to do with linux software RAID. I was only > "thinking loud" :) > Thanks. > > Dealing with device betrayal at a hardware level is a difficult problem. I'm under the impression the md driver is very intolerant of write failure and would eject a drive even with a single failed write? It would seem to be disqualifying for RAID. Btrfs still tolerates many errors, read and write, so it can still be a problem there too. But yes it does have an independent way to unambiguously determine whether file system metadata, or extent data, is corrupt. It also often keeps two copies of metadata (the file system itself). Another option (read-only) is dm-verity, but that is not RAID, it uses forward error correction and cryptographic hash verification. -- Chris Murphy ^ permalink raw reply [flat|nested] 46+ messages in thread
* Re: Filesystem corruption on RAID1 2017-07-14 1:48 ` Chris Murphy @ 2017-07-14 7:22 ` Roman Mamedov 0 siblings, 0 replies; 46+ messages in thread From: Roman Mamedov @ 2017-07-14 7:22 UTC (permalink / raw) To: Chris Murphy; +Cc: Linux-RAID On Thu, 13 Jul 2017 19:48:29 -0600 Chris Murphy <lists@colorremedies.com> wrote: > Btrfs still tolerates many errors, read and write Actually it was Btrfs which saved me back then. Btrfs was making two copies of metadata blocks and restored corrupted copies from good ones, and also signaled me that user files were also affected (via data checksums). FS checksums do work, and if you have redundancy for the corrupted part (such as metadata DUP by default, or data DUP (unusual) or data RAID1), allow the FS to sustain through corruptions, including hardware-caused ones. -- With respect, Roman ^ permalink raw reply [flat|nested] 46+ messages in thread
end of thread, other threads:[~2017-09-01 23:14 UTC | newest] Thread overview: 46+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2017-07-13 15:35 Filesystem corruption on RAID1 Gionatan Danti 2017-07-13 16:48 ` Roman Mamedov 2017-07-13 21:28 ` Gionatan Danti 2017-07-13 21:34 ` Reindl Harald 2017-07-13 22:34 ` Gionatan Danti 2017-07-14 0:32 ` Reindl Harald 2017-07-14 0:52 ` Anthony Youngman 2017-07-14 1:10 ` Reindl Harald 2017-07-14 10:46 ` Gionatan Danti 2017-07-14 10:58 ` Reindl Harald 2017-08-17 8:23 ` Gionatan Danti 2017-08-17 12:41 ` Roger Heflin 2017-08-17 14:31 ` Gionatan Danti 2017-08-17 17:33 ` Wols Lists 2017-08-17 20:50 ` Gionatan Danti 2017-08-17 21:01 ` Roger Heflin 2017-08-17 21:21 ` Gionatan Danti 2017-08-17 21:23 ` Gionatan Danti 2017-08-17 22:51 ` Wols Lists 2017-08-18 12:26 ` Gionatan Danti 2017-08-18 12:54 ` Roger Heflin 2017-08-18 19:42 ` Gionatan Danti 2017-08-20 7:14 ` Mikael Abrahamsson 2017-08-20 7:24 ` Gionatan Danti 2017-08-20 10:43 ` Mikael Abrahamsson 2017-08-20 13:07 ` Wols Lists 2017-08-20 15:38 ` Adam Goryachev 2017-08-20 15:48 ` Mikael Abrahamsson 2017-08-20 16:10 ` Wols Lists 2017-08-20 23:11 ` Adam Goryachev 2017-08-21 14:03 ` Anthony Youngman 2017-08-20 19:11 ` Gionatan Danti 2017-08-20 19:03 ` Gionatan Danti 2017-08-20 19:01 ` Gionatan Danti 2017-08-31 22:55 ` Robert L Mathews 2017-09-01 5:39 ` Reindl Harald 2017-09-01 23:14 ` Robert L Mathews 2017-08-20 23:22 ` Chris Murphy 2017-08-21 5:57 ` Gionatan Danti 2017-08-21 8:37 ` Mikael Abrahamsson 2017-08-21 12:28 ` Gionatan Danti 2017-08-21 14:09 ` Anthony Youngman 2017-08-21 17:33 ` Chris Murphy 2017-08-21 17:52 ` Reindl Harald 2017-07-14 1:48 ` Chris Murphy 2017-07-14 7:22 ` Roman Mamedov
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.