On 2020/7/1 上午8:51, Marc Lehmann wrote:
> Hi!
> 
> I have a server with multiple btrfs filesystems and some moderate-sized
> dmcache caches (a few million blocks/100s of GBs).
> 
> When the server has an unclean shutdown, dmcache treats all cached blocks
> as dirty. This has the effect of extremely slow I/O, as dmcache basically
> caches a lot of random I/O, and writing these blocks back to the rotating
> disk backing store can take hours. This, I think, is related to the
> problem.
> 
> When the server is in this condition, then all btrfs filesystems on slow
> stores (regardless of whether they use dmcache or not) fail their first
> mount attempt(s) like this:
> 
>    [  173.243117] BTRFS info (device dm-7): has skinny extents
>    [  864.982108] BTRFS error (device dm-7): open_ctree failed
> 
> Recent kernels sometimes additionally fail like this (super_total_bytes):
> 
>    [  867.721885] BTRFS info (device dm-7): turning on sync discard
>    [  867.722341] BTRFS info (device dm-7): disk space caching is enabled
>    [  867.722691] BTRFS info (device dm-7): has skinny extents
>    [  871.257020] BTRFS error (device dm-7): super_total_bytes 858976681984 mismatch with fs_devices total_rw_bytes 1717953363968
>    [  871.257487] BTRFS error (device dm-7): failed to read chunk tree: -22
>    [  871.269989] BTRFS error (device dm-7): open_ctree failed

This looks like an old fs with some bad accounting numbers.

Have you tried btrfs rescue fix-device-size?

Thanks,
Qu
> 
> all the filesystems in question are mounted twice during normal boots,
> with diferent subvolumes, and systemd parallelises these mounts. This might
> play a role in these failures.
> 
> Simply trying to mount the filesystems again then (usually) succeeds with
> seemingly no issues, so these are spurious mount failures. These repeated
> mount attewmpts are also much faster, presumably because a lot of the data
> is already in memory.
> 
> As far as I am concerned, this is 100% reproducible (i.e. it happens on every
> unclean shutdown). It also happens on "old" (4.19 era) filesystems as well as
> on filesystems that have never seen anything older than 5.4 kernels.
> 
> It does _not_ happen with filesystems on SSDs, regardless of whether they
> are mounted multiple times or not. It does happen to all filesystems that
> are on rotating disks affected by dm-cache writes, regardless of whether
> the filesystem itself uses dmcache or not.
> 
> The system in question is currently running 5.6.17, but the same thing
> happens with 5.4 and 5.2 kernels, and it might have happened with much
> earlier kernels as well, but I didn't have time to report this (as I
> secretly hoped newer kernels would fix this, and unclean shutdowns are
> rare).
> 
> Example btrfs kernel messages for one such unclean boot. This involved
> normal boot, followed by unsuccessfull "mount -va" in the emergency shell
> (i.e. a second mount fasilure for the same filesystem), followed by a
> successfull "mount -va" in the shell.
> 
> [  122.856787] BTRFS: device label LOCALVOL devid 1 transid 152865 /dev/mapper/cryptlocalvol scanned by btrfs (727)
> [  173.242545] BTRFS info (device dm-7): disk space caching is enabled
> [  173.243117] BTRFS info (device dm-7): has skinny extents
> [  363.573875] INFO: task mount:1103 blocked for more than 120 seconds.
> the above message repeats multiple times, backtrace &c has been removed for clarity
> [  484.405875] INFO: task mount:1103 blocked for more than 241 seconds.
> [  605.237859] INFO: task mount:1103 blocked for more than 362 seconds.
> [  605.252478] INFO: task mount:1211 blocked for more than 120 seconds.
> [  726.069900] INFO: task mount:1103 blocked for more than 483 seconds.
> [  726.084415] INFO: task mount:1211 blocked for more than 241 seconds.
> [  846.901874] INFO: task mount:1103 blocked for more than 604 seconds.
> [  846.916431] INFO: task mount:1211 blocked for more than 362 seconds.
> [  864.982108] BTRFS error (device dm-7): open_ctree failed
> [  867.551400] BTRFS info (device dm-7): turning on sync discard
> [  867.551875] BTRFS info (device dm-7): disk space caching is enabled
> [  867.552242] BTRFS info (device dm-7): has skinny extents
> [  867.565896] BTRFS error (device dm-7): open_ctree failed
> [  867.721885] BTRFS info (device dm-7): turning on sync discard
> [  867.722341] BTRFS info (device dm-7): disk space caching is enabled
> [  867.722691] BTRFS info (device dm-7): has skinny extents
> [  871.257020] BTRFS error (device dm-7): super_total_bytes 858976681984 mismatch with fs_devices total_rw_bytes 1717953363968
> [  871.257487] BTRFS error (device dm-7): failed to read chunk tree: -22
> [  871.269989] BTRFS error (device dm-7): open_ctree failed
> [  872.535935] BTRFS info (device dm-7): disk space caching is enabled
> [  872.536438] BTRFS info (device dm-7): has skinny extents
> 
> Example fstab entries for the mounts above:
> 
> /dev/mapper/cryptlocalvol       /localvol       btrfs           defaults,nossd,discard                  0       0
> /dev/mapper/cryptlocalvol       /cryptlocalvol  btrfs           defaults,nossd,subvol=/                 0       0
> 
> I don't need assistance, I merely write this in the hope of btrfs being
> improved by this information.
>