[sheepdog] [Qemu-devel] [PATCH 0/3] sheepdog: support online snapshot from qemu-img

[Stefan Hajnoczi]

On Thu, Apr 25, 2013 at 07:44:29AM -0600, Eric Blake wrote:
> On 04/25/2013 04:37 AM, MORITA Kazutaka wrote:
> > Currently, we can take sheepdog snapshots of running VMs only from the
> > qemu monitor.  This series allows taking online snapshots from
> > qemu-img.
> 
> Is that even safe?  We specifically recommend that users NOT use
> qemu-img on any image that is in-use by a qemu process, and instead only
> use qemu monitor commands; since qemu-img may see inconsistent state,
> and worse, any writes that qemu-img do may cause qemu to see
> inconsistent state and cause corruption to the running guest.

It's safe in the same way that multiple hosts can access the same NFS
export.  All I/O is going through the same NFS server and is therefore
kind of consistent ;).

The problem that qcow2 has is that two programs would overwrite metadata
in the file, possibly stepping on each other's feet and corrupting the
image metadata.

Sheepdog can handle snapshot requests coming from different clients.

Users can still shoot themselves in the foot if they start doing things
like reading or writing image data while the guest is running.

Stefan