[sheepdog-users] About snapshots

[Liu Yuan]

On Fri, Aug 16, 2013 at 11:22:17AM +0200, Ing. Luca Lazzeroni - Trend Servizi Srl wrote:
> Il giorno 16/ago/2013, alle ore 11:03, Liu Yuan <namei.unix at gmail.com> ha scritto:
> 
> > On Fri, Aug 16, 2013 at 10:48:17AM +0200, Ing. Luca Lazzeroni - Trend Servizi Srl wrote:
> >> Hi,
> >> I'm trying to use external snapshot feature of QEMU/libVirt with sheepdog baked vdi to take VDI live backups without using sheepdog snapshots.
> >> This is to avoid the problem of disk-space unreleased on snapshot deletion.
> > 
> > Maybe you can simply use qemu-img sheepdog:image local_iamge to read the image
> > out of cluster?
> > 
> 
> Yes, I could, but I need "live" backups, so I can't stop the running vm and backing up the VDI of a running VM can lead to data corruption,
> 
> > The release of unused objects from deleted snapshots will be supported in the
> > up comming series of v0.7.x, I guess in about one month or two.
> > 
> 
> Good, but I'm trying to find out a "tricky" solution in the meantime :-)

I think your approach can work. Just have images backed by sheepdog formated as
qcow2. (qemu-img create -f qcow2 sheepdog:image).

> >> The main problem is that virsh seems to fail to create external snapshot (baked on qcow2 files on local server filesystem) on vm running within sheepdog, probably due to qemu inability to create disk-mirror across the 2 storage technologies (sheepdog / local fs).
> >> 
> >> Is anyone trying to do this ? Or is this the wrong way to take full backups ?
> > 
> > I think 'dog cluster snapshot' will serve you better for 'full backups'
> > 
> 
> Does this command stop/block VM during the backup ?

Umm, this command just backup the snapthos vdi that are already taken in the
cluster.

>
> If this creates snapshot on cluster, I think it created automatically snapshot of any vdi on it. 

For now, we don't create snapshots on lively running vdi. But we might should do
it. Prabably we'll take this approach to snapshot the cluster later.

> Are they occupying space on cluster after backup terminated ?
>

If we take snapshot of running vdi while do 'cluster snapshot', yes, we'll
create new snapshots, so we'll create some meta data for these snapshots to
consume some space in the cluster, but the space used is pratically very few,
that is, only 4M for each vdi at the moment of doing 'cluster snapshot'.

Thanks
Yuan