[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM
g.danti at assyoma.it
Thu Mar 1 12:48:09 UTC 2018
On 01/03/2018 12:23, Zdenek Kabelac wrote:
> In general - for extX it's remount read-only upon error - which works
> for journaled metadata - if you want same protection for 'data' you need
> to switch to rather expensive data journaling mode.
> For XFS there is now similar logic where write error on journal stops
> filesystem usage - look far some older message (even here in this list)
> it's been mentioned already few times I guess...
Yes, we discussed here the issue. If I recall correctly, XFS journal is
a circular buffer which will be always written to already-allocated
chunks. From my tests (June 2017) it was clear that failing async
writes, even with errorwhenfull=y, did not always trigger a prompt XFS
stop (but the filesystem eventally shut down after some more
> Depends on use-case - if you take snapshots of your thin volume, this
> likely has will not help you with recovery at all.
> If your thin-volumes are rather standalone only occasionally modified
> 'growing' fs images (so no trimming ;)) - then with this metadata
> backup there can be some small chance you would be able to obtain some
> 'usable' mappings of chunks to block device layout...
> Personally I'd not recommend to use this at all unless you know rather
> low-level details how this whole thing works....
Ok, I realized that and stopped using it for anything but testing.
> Unfreezed filesystem is simply not usable...
I was speaking about unfreezed thin metadata snapshot - ie:
reserve_metadata_snap *without* a corresponding release_metadata_snap.
Will that cause problems?
> Clearly Startis is not a topic for lvm2 at all ;) that's all I'm going
> to say about this....
I think VDO is a fruit of Permabit acquisition, right? As it implements
it's own thin provisioning, will thinlvm migrate to VDO or it will
continue to use the current dmtarget?
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