bcirvin,
you proposed "something to allow us to try to pull data from a failed pool".
Yes and no. 'Yes' as a pragmatic solution; 'no' for what ZFS was 'sold' to be:
the last filesystem mankind would need. It was conceived as a filesystem that
does not need recovery, due to its guaranteed consistent states on the/any
drive - or better: at any moment. If this was truly the case, a recovery
program was not needed, and I don't think SUN will like one neither.
It also is more then suboptimal to prevent caching as proposed by others; this
is but a very ugly hack.
Again, and I have yet to receive comments on this, the original poster claimed
to have done a proper flash/sync, and left a 100% consistent file system behind
on his drive. At reboot, the pool, the higher entity, failed miserably.
Of course, now one can conceive a program that scans the whole drive, like in
the good ole days on ancient file systems to recover all those 100% correct
file system(s).
Or, one could - as proposed - add an Überblock, like we had the FAT-mirror in
the last millennium.
The alternative, and engineering-wise much better solution, would be to
diagnose the weakness on the contextual or semantical level: Where 100%
consistent file systems cannot be communicated to by the operating system. This
- so it seems - is (still) a shortcoming of the concept of ZFS. Which might be
solved by means of yesterday, I agree.
Or, by throwing more work into the level of the volume management, the pools.
Without claiming to have the solution, conceptually I might want to propose to
do away with the static, look-up-table-like structure of the pool, as stored in
a mirror or Überblock. Could it be feasible to associate pools dynamically?
Could it be feasible, that the filesystems in a pool create a (new) handle once
they are updated in a consistent manner? And when the drive is plugged/turned
on, the software simply collects all the handles of all file systems on that
drive? Then the export/import is possible, but not required any longer, since
the filesystems form their own entities. They can still have associated
contextual/semantic (stored) structures into which they are 'plugged' once the
drive is up; if one wanted to ('logical volume'). But with or without, the pool
would self-configure when the drive starts by picking up all file system
handles.
Uwe
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