[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM

Gionatan Danti 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....

OK :p

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?

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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