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

[Gionatan Danti]

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 
writes/minutes).

> 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
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