snapshot storage use

snapshot storage use

[Travis Rhoden]

Hey folks,

Sorry if this info is covered elsewhere -- I searched but was unable
to find a description of what I wanted.

Roughly how much storage is used by snapshots?  Is it really just
marking the version of certain objects, such that multiple snapshots
of the same (unchanged) object consume essentially no new space?
Correspondingly, once that object is finally modified, it is copied
and the snapshots still point to the original (old) copy?  Is this
done by using hardlinks or some other method?

Just wanted the 10,000 foot view explanation.  =)

Thanks,

 - Travis

Re: snapshot storage use

[Josh Durgin]

On 07/17/2012 11:11 AM, Travis Rhoden wrote:
> Hey folks,
>
> Sorry if this info is covered elsewhere -- I searched but was unable
> to find a description of what I wanted.
>
> Roughly how much storage is used by snapshots?  Is it really just
> marking the version of certain objects, such that multiple snapshots
> of the same (unchanged) object consume essentially no new space?
> Correspondingly, once that object is finally modified, it is copied
> and the snapshots still point to the original (old) copy?  Is this
> done by using hardlinks or some other method?

Yes, snapshots are copy-on-write at the object level. On the osd, this
is either an efficient clone (btrfs) or a full copy of the object 
(non-btrfs).

>
> Just wanted the 10,000 foot view explanation.  =)
>
> Thanks,
>
>   - Travis