Re: btrfs_log2phys: cannot lookup extent mapping

[David Hanke <hanke.list@ece.wisc.edu>]

Belated thanks to Duncan, Adam, Austin and Xin for your replies and 
thank you to everyone who's working on btrfs!

Sincerely,

David


On 12/21/16 08:50, David Hanke wrote:
> Hi Duncan,
>
> Thank you for your reply. If I've emailed the wrong list, please let 
> me know. What I hear you saying, in short, is that btrfs is not yet 
> fully stable but current 4.x versions may work better. I'm willing to 
> upgrade, but I'm told that the upgrade process may result in total 
> failure, and I'm not sure I can trust the contents of the volume 
> either way. Given that, it seems I must backup the backup, erase and 
> start over. What would you do?
>
> Thank you,
>
> David
>
>
> On 12/20/16 17:24, Duncan wrote:
>> David Hanke posted on Tue, 20 Dec 2016 09:52:25 -0600 as excerpted:
>>
>>> I've been using a btrfs-based volume for backups, but lately the
>>> system's been filling the syslog with errors like "btrfs_log2phys:
>>> cannot lookup extent mapping for 7129125486592" at the rate of hundreds
>>> per second. (Please see output below for more details.) Despite the
>>> errors, the files I've looked at appear to be written and read
>>> successfully.
>>>
>>> I'm wondering if the contents of the volume are trustworthy and whether
>>> this problem is resolvable without backing up, erasing and starting
>>> over?
>>>
>>> Thank you!
>>>
>>> David
>>>
>>>
>>> # uname -a
>>> Linux backup2 3.0.101.RNx86_64.3 #1 SMP Wed Apr 1 16:02:14 PDT 2015
>>> x86_64 GNU/Linux
>>>
>>> # btrfs --version
>>> Btrfs v3.17.3
>> FWIW...
>>
>> [TL;DR: see the four bottom line choices, at the bottom.]
>>
>> This is the upstream btrfs development and discussion list for a
>> filesystem that's still stabilizing (that is, not fully stable and
>> mature) and that remains under heavy development and bug fixing.  As
>> such, list focus is heavily forward looking, with an extremely strong
>> recommendation to use current kernels (and to a lessor extent btrfs
>> userspace) if you're going to be running btrfs, as these have all the
>> latest bugfixes.
>>
>> Put a different way, the general view and strong recommendation of the
>> list is that because btrfs is still under heavy development, with bug
>> fixes, some more major than others, every kernel cycle, while we
>> recognize that choosing to run old and stale^H^Hble kernels and 
>> userspace
>> is a legitimate choice on its own, that choice of stability over support
>> for the latest and greatest, is viewed as incompatible with choosing to
>> run a still under heavy development filesystem.  Choosing one OR the
>> other is strongly recommended.
>>
>> For list purposes, we recommend and best support the last two kernel
>> release series in two tracks, LTS/long-term-stable, or current release
>> track.  On the LTS track, that's the LTS 4.4 and 4.1 series.  On the
>> current track, 4.9 is the latest release, so 4.9 and 4.8 are best
>> supported.
>>
>> Meanwhile, it's worth keeping in mind that the experimental label and
>> accompanying extremely strong "eat your babies" level warnings weren't
>> peeled off until IIRC 3.12 or so, meaning anything before that is not
>> only ancient history in list terms, but also still labeled as "eat your
>> babies" level experimental.  Why anyone choosing to run an ancient eat-
>> your-babies level experimental version of a filesystem that's now rather
>> more stable and mature, tho not yet fully stabilized, is beyond me.  If
>> they're interested in newer filesystems, running newer and less buggy
>> versions is reasonable; if they're interested in years-stale level of
>> stability, then running such filesystems, especially when still labeled
>> eat-your-babies level experimental back then, seems an extremely odd
>> choice indeed.
>>
>> Of course, on-list we do recognize that various distros did and do offer
>> support at some level for older than list-recommended version btrfs, in
>> part because they backport fixes from newer versions.  However, because
>> we're forward development focused we don't track what patches these
>> distros may or may not have backported and thus aren't in a good 
>> position
>> to provide good support for them.  Instead, users choosing to use such
>> kernels are generally asked to choose between upgrading to something
>> reasonably supportable on-list if they wish to go that route, or 
>> referred
>> back to their distros for the support they're in a far better 
>> position to
>> offer, since they know what they've backported and what they haven't,
>> while we don't.
>>
>> As for btrfs userspace, the way btrfs works, during normal runtime,
>> userspace primarily calls the kernel to do the real work, so userspace
>> version isn't as big a deal unless you're trying to use a feature only
>> supported by newer versions, except that if it's /too/ old, the 
>> impedance
>> mismatch between the commands as they were then and the commands in
>> current versions makes support rather more difficult.  However, once
>> there's a problem, then the age of userspace code becomes more vital, as
>> then it's actually the userspace code doing the work, and only newer
>> versions of btrfs check and btrfs restore, for instance, can detect and
>> fix problems where code has only recently been added to do so.
>>
>> In general, then, with btrfs-progs releases and versioning synced to 
>> that
>> of the kernel, a reasonable rule of thumb is to run userspace of a
>> similar version to your kernel, tho unless you're experiencing problems,
>> getting a version or two behind on your userspace isn't a big deal.  
>> That
>> way, userspace command formats and output will be close enough to 
>> current
>> for easier support, and if there's a fix for a specific problem you've
>> posted in newer userspace, the problem and fix are still fresh enough in
>> people's minds that someone will probably recognize it and point out 
>> that
>> a newer version can handle that, and you can worry about upgrading to 
>> the
>> latest and greatest at that point.
>>
>> So bottom line, you have four choices:
>>
>> 1) Upgrade to something reasonably current to get better on-list 
>> support.
>>
>> This would be LTS kernels 4.4 preferred, or 4.1, acceptable, or current
>> kernels 4.9 or 4.8, and similarly versioned userspace, so no older than
>> btrfs-progs 4.0.
>>
>> 2) Choose to stay with your distro's versions and get support from them.
>>
>> Particularly if you are already paying for that support, might as well
>> use it.
>>
>> 3) Recognize the fundamental incompatibility between wanting to run old
>> and stale/stable for the stability it is supposed to offer, and wanting
>> to run a still under heavy development not fully stable and mature
>> filesystem like btrfs, and either switch to a more stable and mature
>> filesystem that better meets your needs for those qualities, or upgrade
>> to a distro or distro version that better meets your needs for current
>> software better supported by current upstreams like this btrfs list.
>>
>> 4) Stay with what you have, and muddle through as best you can.
>>
>> After all, it's not like we /refuse/ to offer support to btrfs that old,
>> if we recognize a problem that we know can be fixed by code that old
>> we'll still tell you, and if we know there's a fix in newer versions
>> we'll still tell you and try to point you at the appropriate patch for
>> you to apply to your old version if possible, but we simply recognize
>> that for something that old, our support will be rather limited, at 
>> best.
>>
>> But it remains your system and your data, so your choice, even if it's
>> against everything we normally recommend.
>>
>>
>> Finally, a personal disclosure.  I'm a btrfs user and list regular, 
>> not a
>> dev.  As such, my own answers will rarely get code-level technical or
>> point to specific patches, but because I /am/ a regular, I can still
>> answer the stuff that comes up regularly, leaving the real devs and more
>> expert replies to cover detailed content that's beyond me.  So while 
>> it's
>> quite possible someone else will recognize a specific bug and be able to
>> point you toward a specific fix, tho honestly I don't expect it for
>> something as old as what you're posting about, general list-recommended
>> upgrades and alternatives for people posting with positively ancient
>> versions is squarely within my reply territory. =:^)
>>
>
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