this post was submitted on 13 Sep 2023
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The filesystem metadata comes with 2 copies that can heal each other, and Copy-on-Write protects against power loss. The filesystem itself should be bulletproof.
I feel like people reporting data loss on BTRFS are unaware that at least BTRFS is actually measuring the data loss. Bitrot is not rare, especially with how big our drives are getting. If you care about your data it should be backed up and/or RAIDed. Ext4 has no idea if your data is still intact - that's not the same as no data loss.
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What deduplication program did you use? Deduplication is not technically an end-to-end supported feature, and depending on how the third-party program implemented it there could be issues earlier in the pipeline. I'm also not sure how a RAM bit flip would interact in this scenario - I know ZFS checks the file checksum several times during transaction but I don't know how often BTRFS does.
The problem is that there are a lot of people online reporting vague problems with BTRFS, but all reports have little info on how they were actually caused and are not able to be reproduced. There is no solution if we're operating under these rules, other than to completely stop using BTRFS out of pure superstition. If there are bugs we need to be able to point to the bugs in order to fix them. As I said before, this problem you had would not have even been detected by Ext4, so I think error reporting is biased against a FS that actually checks its work. W/r/t to checking work, I think ZFS gets away with a lot more because it's normally run in RAID setups, where healing happens automatically. BTRFS, lacking RAID5/6 support, is usually just run on a single drive, and any data integrity error becomes a target of frustration as soon as it happens.
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I'm interested to see that reported somewhere - the duperemove repo might be a good starting point as that's generally the standard BTRFS dedupe solution. I don't currently see any issues on the GitHub repo about corruption (or at least the last one was 7 years ago). Again, I'm not sure if a RAM bit flip could cause this during a dedupe. Just because you scrubbed, deduped, and scrubbed again doesn't mean there wasn't a bit flip during the dedupe.
As for
btrfs-check
vsfsck
, there are just way fewer things that need to be repaired in BTRFS and ZFS because they are copy-on-write (ZFS doesn't even have afsck
at all!). Because Ext4 is not Copy-On-Write, it's highly vulnerable to powerloss events, and anfsck
is required to replay the journal when this happens. BTRFS and ZFS make atomic COW transactions and will never be in a state of corruption on power loss. The other part offsck
is repairing the filesystem, which BTRFS and ZFS do throughscrub
and/or auto-heal on read instead. ZFS and BTRFS keep multiple copies of the filesystem metadata so that it can auto-repair itself while online.btrfs check
is not something that should be used lightly, and I've seen a lot of people just runbtrfs-check --repair
expecting the same behavior asfsck
, then wonder why they ended up with a broken filesystem.