On 7/9/20 2:24 PM, Eric Sandeen wrote:
<50 runs later on btrfs>

16 readonly mounts failed (32% failure rate)
Within the successful mounts, 1 or more files were unreachable in 30 attempts.
Across all 50 attempts, 7720 files were lost.

Is that better than ext4, and will ext4 need fsck just to be able to mount?

<50 runs later on ext4, same strategy>

zero mount failures for ext4.
Within the successful mounts, 1 or more files were unreachable in 2 attempts.
Across all 50 attempts, 48 files were lost.

But for that test to be meaningful, you need to check that the files that ext4 recovers are actually what you expect---after all, if the metadata is damaged and repaired incorrectly, it could point to some random blocks and we'd never know. This is not just theoretical concern---I have seen this type of damage in fsck'ed systems, although I admit it has been long ago. The type of damage might be tricky---for instance part of the file would be correct, but other parts would be wrong, or the file would be truncated.

Btrfs will just give up if it screws up. You could see it as good or bad---after all, if a disk holding your pictures went bad, maybe it is useful to see partially damaged pictures, rather than having the filesystem throw up its hands. On the other hand, btrfs being harsh like that basically sends the message to 'backup or else', which may be the right thing in the end.
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