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by p_l 296 days ago
The lack of traditional 'fsck' is because its operation would be exact same as normal driver operation. The most extreme case involves a very obscure option that lets you explicitly rewind transactions to one you specify, which I've seen used to recover a broken driver upgrade that led to filesystem corruption in ways that most FSCK just barf on, including XFS'

For low-level meddling and recovery, there's a filesystem debugger that understands all parts of ZFS and can help for example identifying previous uberblock that is uncorrupted, or recovering specific data, etc.
1 comments
koverstreet 296 days ago
Rewinding transactions is cool. Bcachefs has that too :)

What happens on ZFS if you lose all your alloc info? Or are there other single points of failure besides the ublock in the on disk format?

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magicalhippo 296 days ago
> What happens on ZFS if you lose all your alloc info?

According to this[1] old issue, it hasn't happened frequently enough to prioritize implementing a rebuild option, however one should be able to import the pool read-only and zfs send it to a different pool.

As far as I can tell that's status quo. I agree it is something that should be implemented at some point.

That said, certain other spacemap errors might be recoverable[2].

[1]: https://github.com/openzfs/zfs/issues/3210

[2]: https://github.com/openzfs/zfs/issues/13483#issuecomment-120...

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koverstreet 296 days ago
I take a harder line on repair than the ZFS devs, then :)

If I see an issue that causes a filesystem to become unavailable _once_, I'll write the repair code.

Experience has taught me that there's a good chance I'll be glad I did, and I like the peace of mind that I get from that.

And it hasn't been that bad to keep up on, thanks to lucky design decisions. Since bcachefs started out as bcache, with no persistent alloc info, we've always had the ability to fully rebuild alloc info, and that's probably the biggest and hardest one to get right.

You can metaphorically light your filesystem on fire with bcachefs, and it'll repair. It'll work with whatever is still there and get you a working filesystem again with the minimum possible data loss.

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magicalhippo 296 days ago
As I said I do think ZFS is great, but there are aspects where it's quite noticeable it was born in an enterprise setting. That sending, recreating and restoring the pool is a sufficient disaster recovery plan to not warrant significant development is one of those aspects.

As I mentioned in the other subthread, I do think your commitment to help your users is very commendable.

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koverstreet 296 days ago
Oh, I'm not trying to diss ZFS at all. You and I are in complete agreement, and ZFS makes complete sense in multi device setups with real redundancy and non garbage hardware - which is what it was designed for, after all.

Just trying to give honest assessments and comparisons.

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