[sheepdog-users] Different write speed between vdi

[Liu Yuan]

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.

Thanks
Yuan