Case study: recovery of a corrupted 12 TB multi-device pool

https://github.com/kdave/btrfs-progs/issues/1107

by salt4034 • 4 months ago

116 62 4 months ago

phoronixrly5• 4 months ago

To theal author: did you continue using btrfs after this ordeal? An FS that will not eat (all) your data upon a hard powercycle only at the cost of 14 custom C tools is a hard pass from me no matter how many distros try to push it down my throat as 'production-ready'...

Also, impressive work!

fpoling3• 4 months ago

What are the alternatives to btrfs? At 12 TB data checksums are a must unless the data tolerate bit-rot. And if one wants to stick with the official kernel without out-of-tree modules, btrfs is the only choice.

egorfine• 4 months ago

if one wants to stick with the official kernel without out-of-tree modules

I wonder how could a requirement like that possibly arise. Especially with an obvious exception for zfs.

Joel_Mckay• 4 months ago

Could try ZFS or CephFS... even if several host roles are in VM containers (45Drives has a product setup that way.)

The btrfs solution has a mixed history, and had a lot of the same issues DRBD could get. They are great until some hardware/kernel-mod eventually goes sideways, and then the auto-heal cluster filesystems start to make a lot more sense. Note, with cluster based complete-file copy/repair object features the damage is localized to single files at worst, and folks don't have to wait 3 days to bring up the cluster on a crash.

Best of luck, =3

aktau• 4 months ago

I tried btrfs on three different occasions. Three times it managed to corrupt itself. I'll admit I was too enthousiastic the first time, trying it less than a year after it appeared in major distros. But the latter two are unforgiveable (I had to reinstall my mom's laptop).

I've been using ZFS for my NAS-like thing since then. It's been rock solid ().

(): I know about the block cloning bug, and the encryption bug. Luckily I avoided those (I don't tend to enable new features like block cloning, and I didn't have an encrypted dataset at the time). Still, all in all it's been really good in comparison to btrfs.

phoronixrly5• 4 months ago

lvm offers lvmraid, integrity, and snapshots as one example. It's old unsexy tech, but losing data is not to my taste lately...

stinkbeetle2• 4 months ago

What devices are you talking about, what's the UBER, over what period of time?

RAID and logical block redundancy has scaled to petabytes for years in serious production use, before btrfs was even developed.

Sesse__• 4 months ago

Good thing all disks these days have data checksums, then!

(50TB+ on ext4 and xfs, and no, no bit rot. Yes, I've checked most of it against separate sha256sum files now and then. As long as you have ECC RAM, disks just magically corrupting your data is largely a myth.)

raron• 4 months ago

I think you could use dm-integrity over the raw disks to have checksums and protect against bitrot then you can use mdraid to make a RAID1/5/6 of the virtual blockdevs presented by dm-integrity.

I suspect this is still vulnerable to the write hole problem.

You can add LVM to get snapshots, but this still not an end-to-end copy-on-write solution that btrfs and ZFS should provide.

stinkbeetle2• 4 months ago

Case study: recovery of a severely corrupted 12 TB multi-device pool, plus constructive gap analysis and reference tool set #1107

Please don't be btrfs please don't be btrfs please don't be btrfs...

toaste_• 4 months ago

I mean, the only other option was bcachefs, which might have been funny if this LLM-generated blogpost were written by the OpenClaw instance the developer has decided is sentient:

https://www.reddit.com/r/bcachefs/comments/1rblll1/the_blog_...

But no. It was btrfs.

As a side note, it's somewhat impressive that an LLM agent was able to produce a suite of custom tools that were apparently successfully used to recover some data from a corrupted btrfs array, even ad-hoc.

yjftsjthsd-h4• 4 months ago

It could be ZFS. I'd be much more surprised, but it can still have bugs.

curt15• 4 months ago

Where are all of the ZFS corruption stories? Or are there simply fewer of those?

Neikius2• 4 months ago

Not sure about the stats, but it does feel like there are fewer. So from what I know encryption and sending fs state had bugs in ZFS.

And on btrfs anything above raid1 (5,6 etc) has had very serious bugs. Actually read an opinion somewhere (don't remember where) raid5,6 on btrfs cannot work due to on-disk format being just bad for the case. I guess this is why raid1c3/c4 is being promoted and worked on now?

nubinetwork2• 4 months ago

Most of them are from new features that didn't get a proper shakedown test, like encryption.

abrookewood• 4 months ago

I love ZFS, but have been corrected a couple of times when I said it was bomb proof. Can't remember the details, but it has served me faithfully for 10 years or so? Plus the bugs were pretty niche if I recall correctly.

Edit: found some comments below: ZFS on Linux has had many bugs over the years, notably with ZFS-native encryption and especially sending/receiving encrypted volumes. Another issue is that using swap on ZFS is still guaranteed to hang the kernel in low memory scenarios, because ZFS needs to allocate memory to write to swap.

yjftsjthsd-h4• 4 months ago

This is not a bug report. [...] The goal is constructive, not a complaint.

Er, I appreciate trying to be constructive, but in what possible situation is it not a bug that a power cycle can lose the pool? And if it's not technically a "bug" because BTRFS officially specifies that it can fail like that, why is that not in big bold text at the start of any docs on it? 'Cuz that's kind of a big deal for users to know.

EDIT: From the longer write-up:

Initial damage. A hard power cycle interrupted a commit at generation 18958 to 18959. Both DUP copies of several metadata blocks were written with inconsistent parent and child generations.

Did the author disable safety mechanisms for that to happen? I'm coming from being more familiar with ZFS, but I would have expected BTRFS to also use a CoW model where it wasn't possible to have multiple inconsistent metadata blocks in a way that didn't just revert you to the last fully-good commit. If it does that by default but there's a way to disable that protection in the name of improving performance, that would significantly change my view of this whole thing.

rincebrain2• 4 months ago

As far as I can see, no, the author disabled nothing of the sort that he documented.

I suspect that the author's intent is less "I do not view this as a bug" and more "I do not think it's useful to get into angry debates over whether something is a bug". I do not know whether this is a common thing on btrfs discussions, but I have certainly seen debates to that effect elsewhere.

(My personal favorite remains "it's not a data loss bug if someone could technically theoretically write something to recover the data". Perhaps, technically, that's true, but if nobody is writing such a tool, nobody is going to care about the semantics there.)

yjftsjthsd-h4• 4 months ago

I suspect that the author's intent is less "I do not view this as a bug" and more "I do not think it's useful to get into angry debates over whether something is a bug".

Agreed, and I appreciate the attempt to channel things into a productive conversation.

Retr0id3• 4 months ago

Unless I missed it the writeup never identifies a causal bug, only things that made recovery harder.

rcxdude2• 4 months ago

btrfs's reputation is not great in this regard.

stingraycharles• 4 months ago

As far as I understand, single device and RAID1 is solid, but as soon as you want to do RAID1+0 or RAID5/6 you’re entering dangerous territory with BTRFS.

Retr0id3• 4 months ago

This is obviously LLM output, but perhaps LLM output that corresponds to a real scenario. It's plausible that Claude was able to autonomously recover a corrupted fs, but I would not trust its "insights" by default. I'd love to see a btrfs dev's take on this!

number6• 4 months ago

This is also my first impulse. The second was, if this happened to me, I would not be able to recover it. All the custom c tool talk... If you ask Claude Code it will code something up.

Well that he recovered the disks is amazing in itself. I would have given up and just pulled a backup.

However, I would like to see a Dev saying: why didn't you use the --<flag> which we created for this Usecase

yjftsjthsd-h4• 4 months ago

I was assuming real scenario with heavy LLM help to recover. Would be nice for the author to clarify. And, separately, for BTRFS devs to weigh in, though I'd somewhat prefer to get some indication that it's real before spending their time.

nslsm• 4 months ago

An LLM wouldn't make a mistake like "One paragraph summary"

salt4034• 4 months ago

See this Reddit post for background: https://www.reddit.com/r/ClaudeAI/comments/1sdabux/hats_off_...

TLDR: The user got his filesystem corrupted on a forced reboot; native btrfs tools made the failure worse; the user asked Claude to autonomously debug and fix the problem; after multiple days of debugging, Claude wrote a set of custom low-level C scripts to recover 99.9% of the data; the user was impressed and asked Claude to submit an issue describing the whole thing.

Retr0id3• 4 months ago

Good to know that my claude-dar is still working.

harshreality• 4 months ago

Using DUP as the metadata profile sounds insane.

Changing the metadata profile to at least raid1 (raid1, raid1c3, raid1c4) is a good idea, especially for anyone, against recommendations, using raid5 or raid6 for a btrfs array (raid1c3 is more appropriate for raid6). That would make it very difficult for metadata to get corrupted, which is the lion's share of the higher-impact problems with raid5/6 btrfs.

check:

    btrfs fi df <mountpoint>

convert metadata:

    btrfs balance start -mconvert=raid1c3,soft <mountpoint>

(make sure it's -mconvert — m is for metadata — not -dconvert which would switch profiles for data, messing up your array)

throwaway2709252• 4 months ago

This should be at the top, using metadata DUP on a 3 disk volume is already asking for it, and of course you loose data when you just use it as jbod with data stored only once. Unless this are enterprise disks with capacitors anything can happen when it suddenly looses power. Not the FSes fault.

With the same configuration this can happen with ZFS, bcachefs etc just as well.

rcxdude2• 4 months ago

Will it render the whole filesystem inaccessible and unrepairable on those filesystems as well? One of the issues with btrfs is that it's brittle: failure tends not to cause an inconsistency in the affected part of the filesystem but bring down the whole thing. In general people are a lot more understanding of a power failure resulting in data corruption around the files that are actively being written at the time (there are limits to how much consistency can be achieved here anyway), much less so when the blast radius expands a lot further.

theamk• 4 months ago

Unless this are enterprise disks with capacitors anything can happen when it suddenly looses power. Not the FSes fault.

Most filesystems just get a few files/directories damaged though. ZFS is famous for handling totally crazy things like broken hardware which damages data in-transit. ext4 has no checksum, but at least fsck will drop things into lost+found directory.

The "making all data inaccessible" part is pretty unique to btrfs, and lets not pretend nothing can be done about this.

bombela2• 4 months ago

I still got corrupted metadata with metadata raid1c3 on btrfs on a power loss. I never had this happen with ext4 alone or atop Linux raid.

I want to be clear that losing (meta)data in flight during a power loss is expected. But a broken filesystem after that is definitely not acceptable.

Some postgresql db endedup soft corrupted. Postgresql could not replay its log because btrfs threw IO errors on fsync. That's just plain not acceptable.

c-c-c-c-c• 4 months ago

Added to my list of reasons to never use btrfs in production.

jamesnorden• 4 months ago

People swear btrfs is "safe" now, but I've personally been bitten by data corruption more than once, so I stay away from it now.

Avamander• 4 months ago

I installed Fedora on BTRFS using their installer and I lost that partition entirely. Couldn't wrestle it back to life to even copy stuff off it.

I think what happened was that the machine ran out of battery in suspend, but an unclean shutdown shouldn't cause such a deep corruption.

duskdozer• 4 months ago

Welp. Guess I need to figure out another fs to use for a few drives in a nonraid pool I haven't gotten around to setting up yet. I forget why zfs seemed out. xfs?

Filligree• 4 months ago

ZFS is out because the Linux developers refuse to cooperate by providing the hooks it would need to avoid duplicating the disk cache.

That’s the only real reason. There are some papercuts, but they don’t compare to the risks described in this article.

throwaway2709252• 4 months ago

A hard power cycle on a 3 device pool (data single, metadata DUP, DM-SMR disks) left the extent tree and free space tree in a state that no native repair path could resolve.

As a ZFS wrangler by day:

People in this thread seem to happily shit on btrfs here but this seems to be very much not like a sane, resilient configuration no matter the FS. Just something to keep in mind.

scottlamb2• 4 months ago

Might be true, but I don't see any aspect of that which is relevant to this event:

* Data single obviously means losing a single drive will cause data loss, but no drive was actually lost, right?

* Metadata DUP (not sure if it's across 2 disks or all 3) should be expected to be robust, I'd expect?

* I certainly eye DM-SMR disks with suspicion in general, but it doesn't sound like they were responsible for the damage: "Both DUP copies of several metadata blocks were written with inconsistent parent and child generations."

zootboy2• 4 months ago

Metadata DUP (not sure if it's across 2 disks or all 3) should be expected to be robust, I'd expect?

No. DUP will happily put both copies on the same disk. You would need to use RAID1 (or RAID1c3 for a copy on all disks) if you wanted a guarantee of the metadata being on multiple disks.

lnx01• 4 months ago

bulletproof/bulletproof/bulletproof .... Gemini LLM

londons_explore• 4 months ago

Btrfs allows migration from ext4 with a rather good rollback strategy...

Post-migration, a complete disk image of the original ext4 disk will exist within the new filesystem, using no additional disk space due to the magic of copy-on-write.

Why isn't the repair process the same? Fix the filesystem to get everything online asap, and leave a complete disk image of the old damaged filesystem so other recovery processes can be tried if necessary.

guilhas• 3 months ago

This reads something between cluless/malicious and genius. Crosses several red lights with a car, smashes the car, rebuilds the car with AI, tells people to cross red lights

Keeps repeating btrfs check --repair . This command is dangerous and warned anywhere as a last resort: if you try to execute it you get a warning; the documentation has a warning; any guide from google tell you not to run it unless all else fails; chatgpt/lechat do not metion it, or note it as last resort. So not sure why he keeps repeating it without any note

Use these tools ONLY if btrfs check --repair segfaults, enters an infinite loop, or leaves the filesystem in worse shape than before.
Timeline of events ... First repair attempts. btrfs check --repair

The guy is recommending people brick their volumes permanently as first resort without any warning

Between using a dup profile and this I would not be surprised a btrfs dev just disregarding all as slop

Pool only mounts with rescue=all,ro, fails to mount RW

Also this is important, the data was not lost. Even though read-only

I don't think I would run this code. Still it would be interesting a btrs dev to have look and comment if there is any value in the code generated. As it would be definitely interesting being able to repair more issues in the pool safely inplace

Case study: recovery of a corrupted 12 TB multi-device pool

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