On 01/05/2019 15:53, Michelle Sullivan wrote:
Paul Mather wrote:
On Apr 30, 2019, at 11:17 PM, Michelle Sullivan <miche...@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.
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.
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?
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
https://sdimitro.github.io/post/zfs-lsm-flushing/
A similar behavior resulted in being a bug:
https://www.reddit.com/r/zfs/comments/97czae/zfs_zdb_space_map_errors_on_unmountable_zpool/
Regards
Steve
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