Re: [librbd] Add interface of get the snapshot size?

[Andrey Korolyov <andrey@xdel.ru>]

On 03/24/2014 05:30 PM, Haomai Wang wrote:
> Hi all,
> 
> As we know, snapshot is a lightweight resource in librbd and we
> doesn't have any statistic informations about it. But it causes some
> problems to the cloud management.
> 
> We can't measure the size of snapshot, different snapshot will occur
> different space. So we don't have way to estimate the resource usage
> of user.
> 
> Maybe we can have a counter to record space usage when volumn created.
> When creating snapshot, the counter is freeze and store as the size of
> snapshot. New counter will assign to zero for the volume.
> 
> Any feedback is appreciate!
> 

I believe that there is a rough estimation over 'rados df'. Per-image
statistics would be awesome, though precise stats will be neither rough
too(# of rbd object clones per volume) or introduce new counter
mechanism. Dealing with discard for the filestore, it looks even more
difficult to calculate right estimation, as with XFS preallocation feature.