Zdenek Kabelac schreef op 28-03-2018 0:17:
Hi
This is why users do open BZ if they would like to see some
enhancement.
Normally cache is integral part of a volume - so it's partially
missing - whole volume is considered to be garbage.
But in 'writethrough' mode there could be likely possible better
recovery.
Of course this case needs usability without --force.
So please open RFE BZ for this case.
It goes into the mess I usually get myself into; if you "dd copy" the
disk containing the origin volume before uncaching it, and then go to
some live session where you only have the new backup copy, but you want
to clean up its LVM,
then you now must fix the VGs in isolation of the cache; I suppose this
is just the wrong order of doing things, but as part of a backup you
don't really want to uncache first, as that requires more work to get it
back to normal after.
So you end up in a situation where the new origin copy has a reference
to the cache disk --- all of this assumes writethrough mode --- and you
need to clear that reference.
However, you cannot, or should not, attach the cache disk again; it
might get effected, and you don't want that, you want it to remain in
its pristine state.
Therefore, you are now left with the task of removing the cache from the
VG, because you cannot actually run vgimportclone while the cache disk
is missing.
The obvious solution is to *also* clone the cache disk and then run
operations on the combined set, but this might not be possible.
Therefore, all that was left was:
vgreduce --remove-missing --force
cd /etc/lvm/archive
cp /etc/lvm/backup/
cd /etc/lvm/backup
vi
" remove cache PV, and change origin to regular linear volume, and add
" the visible tag
vgcfgrestore
# presto, origin is restored as regular volume without the cache
vgimportclone -i
# now have distinct volume group, VG UUID and PV UUID
So the problem is making dd backups of origin, perhaps dd backups should
be avoided, but for some purposes (such as system migration) file copies
are just
more work
in general, and can complicate things as well, for instance if there are
NTFS partitions or whatnot.
And disk images can be nice to have, in any case.
This was the use case basically.
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