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Comment by thatcks | Hacker News Reader

The article is correct but it downplays an important limitation of ZFS scrubs when it talks about how they're different from fsck and chkdsk. As the article says (in different words), ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems. Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems, and as it stands today ZFS doesn't have anything that either checks or corrects these. Sometimes you find them through incorrect results; sometimes you discover they exist through ZFS assertion failures triggering kernel panics.

(We run ZFS in production and have not been hit by these issues, at least not that we know about. But I know of some historical ZFS bugs in this area and mysterious issues that AFAIK have never been fully diagnosed.)

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    "Scrubs differ significantly from traditional filesystem checks. Tools such as fsck or chkdsk examine logical structures and attempt to repair inconsistencies related to directory trees, allocation maps, reference counts, and other metadata relationships. ZFS does not need to perform these operations during normal scrubs because its transactional design ensures metadata consistency. Every transaction group moves the filesystem from one valid state to another. The scrub verifies the correctness of the data and metadata at the block level, not logical relationships."

> ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems

A scrub literally reads the object from disk. And, for each block, the checksums are read up the tree. The object is therefore guaranteed to be correct and consistent at least re: the tree of blocks written.

> Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems

Can you give a more concrete example of what you mean? It sounds like you have some experience with ZFS, but "ZFS doesn't have an fsck" is also some truly ancient FUD, so you will forgive my skepticism.

I'm willing to believe that you request an object and ZFS cannot return that object because of ... a checksum error or a read error in a single disk configuration, but what I have never seen is a scrub that indicates everything is fine, and then reads which don't return an object (because scrubs are just reads themselves?).

Now, are things like pool metadata corruption possible in ZFS? Yes, certainly. I'm just not sure fsck would or could help you out of the same jam if you were using XFS or ext4. AFAIK fsck may repair inconsistencies but I'm not sure it can repair metadata any better than ZFS can?

Imagine a race condition that writes a file node where a directory node should be. You have a valid object with a valid checksum, but it's hooked into the wrong place in your data structure.

> Imagine a race condition that writes a file node where a directory node should be. You have a valid object with a valid checksum, but it's hooked into the wrong place in your data structure.

A few things: 1) Is this an actual ZFS issue you encountered or is this a hypothetical? 2) And -- you don't imagine this would be discovered during a scrub? Why not? 3) But -- you do imagine it would be discovered and repaired by an fsck instead? Why so? 4) If so, wouldn't this just be a bug, like a fsck, not some fundamental limitation of the system?

FWIW I've never seen anything like this. I have seen Linux plus a flaky ALPM implementation drop reads and writes. I have seen ZFS notice at the very same moment when the power dropped via errors in `zpool status`. I do wonder if ext4's fsck or XFS's fsck does the same when someone who didn't know any better (like me!) sets the power management policy to "min_power" or "med_power_with_dipm".

Here's an example: https://www.illumos.org/issues/17734. But it would not be discovered by a scrub because the hashes are valid. Scrubs check hashes, not structure. It would be discovered by a fsck because the structure is invalid. Fscks check structure, not hashes.

They are two different tools, with two different uses.

> Scrubs check hashes, not structure.

How is the structure not valid here? Can you explain to us how an fsck would discover this bug (show an example where an fsck fixed a similar bug) but ZFS could never? The point I take contention with is that missing an fsck is a problem for ZFS, so more specifically can you answer my 4th Q:

>> 4) If so, wouldn't this just be a bug, like (a bug in) fsck, not some fundamental limitation of the system?

So -- is it possible an fsck might discover an inconsistency ZFS couldn't? Sure. Would this be a fundamental flaw of ZFS, which requires an fsck, instead of merely a bug? I'm less sure.

You do seem to at least understand my general contention with the parent's point. However, the parent is also making a specific claim about a bug which would be extraordinary. Parent's claim is this is a bug which a scrub, which is just a read, wouldn't see, but a subsequent read would reveal.

So -- is it possible an fsck might discover this specific kind of extraordinary bug in ZFS, after a scrub had already read back the data? Of that I'm highly dubious.

> Can you show us how an fsck would discover this bug but ZFS could never?

I'd have to read closer to be certain, but if my understanding of it is correct, you'd have orphaned objects in the file system. The orphaned objects would be detectable, but would have correct hashes.

Here's a better example of one where scrub clearly doesn't catch it, but fsck would (and zdb does). The metaslab structure could be checked in theory, but isn't. https://neurrone.com/posts/openzfs-silent-metaslab-corruptio...

> if so, wouldn't this just be a bug, like (a bug in) fsck, not some fundamental limitation of the system?

It's not a bug or a fundamental limitation of the system, it's just that fsck != scrub, and nobody has written the code for fsck. If someone wanted to write the code, they could. I suspect it wouldn't even be particularly hard.

But fsck != scrub, and they catch different things.

Generally, it's possible to have data which is not corrupted but which is logically inconsistent (incorrect).

Imagine that a directory ZAP has an entry that points to a bogus object ID. That would be an example. The ZAP block is intact but its content is inconsistent.

Such things can only happen through a logical bug in ZFS itself, not through some external force. But bugs do happen.

If your search through OpenZFS bugs you will find multiple instances. Things like leaking objects or space, etc. That's why zdb now has support for some consistency checking (bit not for repairs).

> Imagine that a directory ZAP has an entry that points to a bogus object ID. That would be an example. The ZAP block is intact but its content is inconsistent.

The above is interesting and fair enough, but a few points:

First, I'm not sure that makes what seems to be the parent's point -- that scrub is an inadequate replacement for an fsck.

Second, I'm really unsure if your case is the situation the parent is referring to. Parent seems to be indicating actual data loss is occurring. Not leaking objects or space or bogus object IDs. Parent seems to be saying she/he scrubs with no errors and then when she/he tries to read back a file, oops, ZFS can't.

The two obvious examples that come to mind are native encryption bugs and spacemap issues.

Nothing about walking the entire tree of blocks and checking hashes validates the spacemaps - they only come up when you're dealing with allocating new blocks, and there have been a number of bugs where ZFS panics because the spacemaps say something insane, so you wind up needing to readonly import or discard the ZIL because it panics about trying to allocate an already-allocated segment if you import RW - and if your ondisk spacemaps are inconsistent in a way that discarding the ZIL doesn't work around, you would need some additional tool to try and repair this, because ZFS has no knobs for it.

Native encryption issues wouldn't be noticed because scrubbing doesn't attempt to untransform data blocks - you indirectly do that when you're walking the structures involved, but the L0 data blocks don't get decompressed or decrypted, since all your hashes are of the transformed blocks. And if you have a block where the hash in the metadata is correct but it doesn't decrypt, for any reason, scrub won't notice, but you sure will if you ever try to decrypt it.

> The two obvious examples

Appreciate this rincebrain. Know that you know better than most and this certainly covers my 2nd point. I don't imagine these cases cover my first point though? These are not bugs of the type a fsck would catch?

A fsck could pretty readily notice and repair the spacemap inconsistencies - zdb already generates its own spacemaps and compares to reality on import.

If you have the keys, technically nothing stops a fsck from noticing the encryption problems, but yes, usually it wouldn't unless you had some known issue you added special detection for, like when XFS years ago had problems with if you mounted it with inode64 once and then not the next time so the inode numbers would wraparound.

> Can you give a more concrete example of what you mean?

There's been several instances. For example, the send/receive code has had bugs leading to cases[1] where the checksum and hence scrub look fine but the data is not.

edit: the recent block cloning has also had some issues, eg[2][3].

I'm pretty sure it's also possible for hardware errors like bad memory to cause the data to get corrupted but the checksum gets computed on the corrupted data, thus it looks ok when scrubbed.

[1]: https://github.com/openzfs/zfs/issues/4809

[2]: https://github.com/openzfs/zfs/issues/15526

[3]: https://github.com/openzfs/zfs/issues/15933

I like how ZFS doesn’t have “bugs”, it has “defects”.

> There's been several instances.

I think you're missing the 2nd feature to the parent's point that I take issue with, which is this is not just a bug that a scrub wouldn't find, but it must also be a bug which an fsck would find.

The parent's point is -- ZFS should have an fsck tool because an fsck does something ZFS cannot do by other means. I disagree. Yes, ZFS has bugs like any filesystem. However, I'm not sure an fsck tool would make that situation better?

> I think you're missing the 2nd feature to the parent's point that I take issue with

You're right, I did gloss over that point.

I guess it should be noted that a lot of what fsck does[1] on say ext4 is something ZFS does on pool import, like replying the journal (ZIL) or trying older superblocks (uberblocks[2]). In that regard it's acting more like XFS[3] from what I can see, which just exits with 0.

[1]: https://linux.die.net/man/8/fsck.ext4

[2]: https://openzfs.github.io/openzfs-docs/man/master/8/zpool-im...

[3]: https://linux.die.net/man/8/fsck.xfs

> like replying the journal

Sigh... that's of course replaying the journal.

Two examples that I can find are https://github.com/openzfs/zfs/issues/7910, where very old versions of ZFS appear to have quietly written slightly incorrect ACL information, and https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bug/190... where Ubuntu 21.10 shipped with a bug that created corrupted ZFS filesystems. I believe https://www.illumos.org/issues/9847 may be another example of this, although less severe, where ZFS leaked disk space under some circumstances.

Ubuntu shipped with a bug that they introduced by way of a very badly done patch. While I get your point, I don't think it's fair to use Ubuntu as a source - they're grossly incompetent when it comes to handling ZFS.

> Two examples that I can find

I think you may be misreading my point above. I am not arguing ZFS doesn't have bugs. That's nuts. I am arguing that the bug the parent says he has would be an extraordinary bug.

This is not just a bug that a scrub wouldn't find, but also it is a bug which an fsck would find. And it is not just a bug in the spacemaps or other metadata, but the parent's claim is this is a bug which a scrub, which is just a read, wouldn't see, but a subsequent read would reveal.

I am the parent, and any competent fsck should find these issues. Fsck traditionally explicitly verifies claimed free space against actual free space, and in a filesystem with ACLs it should also verify that filesystem level metadata like ACLs is sane, just as fsck verifies (for example) sane inode flags and inode field values. ZFS scrubs explicitly do not verify spacemap consistency or a lot of other sorts of consistency.

ZFS scrubs don't even verify that a filesystem's directory tree is acyclic and can reach every claimed in-use filesystem object, but I'm not aware of ZFS bugs in that area. This is because ZFS's 'metadata' for scrubs is much different than how it works in a traditional filesystem. To phrase it in conventional filesystem terms, ZFS has a big list of all in-use inodes, and it verifies the filesystem checksums by going through this list. The 'filesystem metadata' that a scrub verifies is the structure of this list of in-use inodes, plus some other things around the corners.

A loooong time age (OpenSolaris days) I had a system that had corrupted its zfs. No fsck was available because the developers claimed (maybe still do) that it's unnecessary.

I had to poke around the raw device (with dd and such) to restore the primary superblock with one of the copies (that zfs keeps in different locations on the device). So clearly the zfs devs thought about the possibility of a corrupt superblock, but didn't feel the need to provide a tool to compare the superblocks and restore one from the other copies. That was the point when I stopped trusting zfs.

Such arrogance…

it's still the case even with now openzfs ? what do you trust now ?

That's a fine fit of pique - and I once had an awkward file on one of my zfs pools, about three pools ago - but how does it leave you better off, if you want what zfs offers?

> That's a fine fit of pique

So you're rejecting a story about a real bug because...?

> how does it leave you better off

That's a really mercenary way to look at learning about your tools.

But presumably they take smaller risks around zfs systems than they otherwise would.

> So clearly the zfs devs thought about the possibility of a corrupt superblock, but didn't feel the need to provide a tool to compare the superblocks and restore one from the other copies.

This mailing list post from 2008 talks about using zdb(8) to mark mark certain uberblocks an invalid so another one would be used:

* https://zfs-discuss.opensolaris.narkive.com/Tx4FaUMv/need-he...

ZDB = ZFS debugger. It's been there since the original Solaris release of ZFS.

> That was the point when I stopped trusting zfs.

As opposed to trusting other file systems and volume managers, which do not have checksums, and so you wouldn't even know about the problem in the first place?

That's not using zdb to change anything - it's readonly, all the time. The person reached out and used dd on the disk to corrupt the copies of the uberblock with bad data so that ZFS would be forced to use the older ones (what zpool import -T does, basically, but doing it the hard way).

In my experience[1], the fsck for given filesystem will happily replicate the errors, sometimes in random ways, because often it cannot figure which road to take in face of inconsistency. If anything, OpenZFS built upon that by now documenting the previously deeply hidden option to "rewind" ZFS uberblock if the breakage is recent enough.

[1] I've seen combination of ubuntu bug in packaging (of grub, of all things) and e2fsck nearly wipe a small company from existence, because e2fsck ended up trusting the data it got from superblock when it was not consistent.

> If anything, OpenZFS built upon that by now documenting the previously deeply hidden option to "rewind" ZFS uberblock if the breakage is recent enough.

One of the most "wizardry" moments in my career I've personally witnessed was a deep-level ZFS expert (core OpenZFS developer) we had on retainer come in during a sev0 emergency and rapidly diagnose/rollback a very broken ZFS filesystem to a previous version from a few hours before the incident happened.

This was entirely user error (an admin connected redundant ZFS "heads" to the same JBOD in an incorrect manner so both thought they were primary and both wrote to the disks) that we caught more or less immediately so the damage was somewhat limited. At the time we thought we were screwed and would have to restore from the previous days backup with a multi-day (at best) time to repair.

This was on illumos a few years after the Solaris fork, so I don't think this feature was documented at the time. It certainly was a surprise to me, even though I knew that "in theory" such capability existed. The CLI incantations though were pure wizardry level stuff, especially watching it in real time with terminal sharing with someone who very much knew what they were doing.

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