[sheepdog-users] Different write speed between vdi

[Liu Yuan]

On Mon, Aug 12, 2013 at 02:59:27PM +0800, Liu Yuan wrote:
> On Mon, Aug 12, 2013 at 08:45:45AM +0200, Valerio Pachera wrote:
> > Hi, on my guest named 'backup' I have two vdi: 'backup' and 'backup_data'.
> > 
> > backup       0   10 GB  3.8 GB  0.0 MB 2013-06-27 15:51   19093f     2
> > backup_data     0  1.0 TB  408 MB  502 GB 2013-08-12 08:15   c8d149     2
> > 
> > The first one, as you can guess, contains only the OS.
> > 
> > Write speed:
> >   backup: 15 M/s
> >   backup_data: 6 M/s
> > 
> > The differences between the two are the size and that 'backup_data'
> > has several snapshots (21).
> > 
> > What do you think about that?
> 
> This is what copy-on-write takes from you when it saves you a lot of space by
> sharing objects between snapshots. When a write is executed, sheep will firstly
> copy the shared object and then write on it. So compared with pure write, COW
> write will spend one more read.

One more thing, after objects are copied by the writes, the successive write
won't incure additional read, the performance will the same as normal vdi.
Simply put, only the write for the first time will be affected.

Thanks
Yuan