Re: [Xen-users] Xen LVM2 snapshot backup

[Peter <peter@xxxxxxxxxxxxxxxx>]

On 29/11/11 08:24, David Della Vecchia wrote:
> I'm not sure what you were expecting, thats how snapshots work, there's
> always a downside. All writes go to the snapshot instead of the origin
> disk and snapshots are sparse disks so they have the added performance
> hit of having to allocate the space as needed up to the limit. If you
> create a snap of a 20gb origin disk and specify -L 1G during the snap
> create it will effectively store 1gb worth of changes until its either
> dropped or auto-expanded depending on your settings. I believe if you do
> not specify a size it will create a snap the same size as the origin
> volume. Snapshots are effectively branches of the origin volume in much
> the same way as they are used in typical source control systems (svn,
> cvs, etc).

Not only that, but any write to the source volume requires a copy of the
existing data from the source to the snapshot in addition to the write,
so that amounts to (for each write to source):

1 read from source.
N writes to snapshots (where N is the number of snapshots)
1 write to source.

...so you can easily see how the IO speed can be affected.  The
advantage you get with snapshots is that they are instantaneous, you
don't risk corrupting your source data and your data won't change in the
middle of making a backup or copy if you use a snapshot.  These are
usually considered to outweigh the performance loss.


Peter

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