ZFS...

Wed May 1 23:46:28 UTC 2019

Michelle Sullivan
http://www.mhix.org/
Sent from my iPad

> On 02 May 2019, at 03:39, Steven Hartland <killing at multiplay.co.uk> wrote:
> 
> 
> 
>> On 01/05/2019 15:53, Michelle Sullivan wrote:
>> Paul Mather wrote:
>>>> On Apr 30, 2019, at 11:17 PM, Michelle Sullivan <michelle at sorbs.net> wrote:
>>>> 
>>>> Been there done that though with ext2 rather than UFS..  still got all my data back... even though it was a nightmare..
>>> 
>>> 
>>> Is that an implication that had all your data been on UFS (or ext2:) this time around you would have got it all back?  (I've got that impression through this thread from things you've written.) That sort of makes it sound like UFS is bulletproof to me.
>> 
>> Its definitely not (and far from it) bullet proof - however when the data on disk is not corrupt I have managed to recover it - even if it has been a nightmare - no structure - all files in lost+found etc... or even resorting to r-studio in the even of lost raid information etc..
> Yes but you seem to have done this with ZFS too, just not in this particularly bad case.
> 

There is no r-studio for zfs or I would have turned to it as soon as this issue hit.

> If you imagine that the in memory update for the metadata was corrupted and then written out to disk, which is what you seem to have experienced with your ZFS pool, then you'd be in much the same position.
>> 
>> This case - from what my limited knowledge has managed to fathom is a spacemap has become corrupt due to partial write during the hard power failure. This was the second hard outage during the resilver process following a drive platter failure (on a ZRAID2 - so single platter failure should be completely recoverable all cases - except hba failure or other corruption which does not appear to be the case).. the spacemap fails checksum (no surprises there being that it was part written) however it cannot be repaired (for what ever reason))... how I get that this is an interesting case... one cannot just assume anything about the corrupt spacemap... it could be complete and just the checksum is wrong, it could be completely corrupt and ignorable.. but what I understand of ZFS (and please watchers chime in if I'm wrong) the spacemap is just the freespace map.. if corrupt or missing one cannot just 'fix it' because there is a very good chance that the fix would corrupt something that is actually allocated and therefore the best solution would be (to "fix it") would be consider it 100% full and therefore 'dead space' .. but zfs doesn't do that - probably a good thing - the result being that a drive that is supposed to be good (and zdb reports some +36m objects there) becomes completely unreadable ...  my thought (desire/want) on a 'walk' tool would be a last resort tool that could walk the datasets and send them elsewhere (like zfs send) so that I could create a new pool elsewhere and send the data it knows about to another pool and then blow away the original - if there are corruptions or data missing, thats my problem it's a last resort.. but in the case the critical structures become corrupt it means a local recovery option is enabled.. it means that if the data is all there and the corruption is just a spacemap one can transfer the entire drive/data to a new pool whilst the original host is rebuilt... this would *significantly* help most people with large pools that have to blow them away and re-create the pools because of errors/corruptions etc... and with the addition of 'rsync' (the checksumming of files) it would be trivial to just 'fix' the data corrupted or missing from a mirror host rather than transferring the entire pool from (possibly) offsite....
> 
> From what I've read that's not a partial write issue, as in that case the pool would have just rolled back. It sounds more like the write was successful but the data in that write was trashed due to your power incident and that was replicated across ALL drives.
> 

I think this might be where the problem started.. it was already rolling back from the first power issue (it did exactly what was expected and programmed, it rolled back 5 seconds.. which as no-one had write access to it from the start of the resilver I really didn’t care as the only changes were the resilver itself.). Now you assertion/musing maybe correct...  all drives got trashed data.. I think not but unless we get into it and examine it I think we won’t know.  What I do know is in the second round -FfX wouldn’t work, I used zdb to locate a “LOADED” MOS and used -t <txg> to import.. the txg number was 7 or 8 from current so just outside of the -X limit (going off memory here, so could have been more, but I remember it was just past the switch limit.)

> To be clear this may or may not be what your seeing as you don't see to have covered any of the details of the issues your seeing and what in detail steps you have tried to recover with?

There have been many steps over the last month.. and some of which I may have made it from very difficult to recover to non recoverable now... though the only writes is what the kernel does as have not got it (the dataset) mounted at any time, even though it has imported.

> 
> I'm not saying this is the case but all may not be lost depending on the exact nature of the corruption.
> 
> For more information on space maps see:
> https://www.delphix.com/blog/delphix-engineering/openzfs-code-walk-metaslabs-and-space-maps

This is something I read a month ago, along with multiple other articles on the same blog, including https://www.delphix.com/blog/openzfs-pool-import-recovery

Which I might add got me from non importable to importable but not mountable.

I have *not* attempted to bypass the checksum line for spacemap load to date as I see that as a possible way to make the problem worse.

> https://sdimitro.github.io/post/zfs-lsm-flushing/

Not read this.

> 
> A similar behavior resulted in being a bug:
> https://www.reddit.com/r/zfs/comments/97czae/zfs_zdb_space_map_errors_on_unmountable_zpool/
> 

Or this.. will go there following “pressing send”.. :)

>     Regards
>     Steve
> _______________________________________________
> freebsd-stable at freebsd.org mailing list
> https://lists.freebsd.org/mailman/listinfo/freebsd-stable
> To unsubscribe, send any mail to "freebsd-stable-unsubscribe at freebsd.org"