[libvirt-users] Re: backup procedure using blockcopy

On 03/18/2013 05:39 AM, Nicolas Sebrecht wrote:

> I'd say that the man page miss the information that these commands can > run with a running guest, dispite the mirroring feature might imply it. Care to write a patch? The virsh man page is generated from libvirt.git:tools/virsh.pod.

> The main drawback I can see is that the hypervisor must have at least as > free disk space than the disks to backup... Or have the path/to/backups > as a remote mount point. Yes, but that's true of any backup operation - you are committing to the disk space for both the live and the backup, in some form or another. Also, disk mirroring to a remote point is fairly easy to set up, by using nbd:... instead of /path/to/file as the destination for the blockcopy, and having an NBD server listening on the remote destination side.

> Now, I wonder if I change of backup strategy and make the remote hosting > the backup mounted locally on the hypervisor (via nfs, iSCSI, sshfs, > etc), should I expect write performance degradation? I mean, does the > running guest wait for underlying both mirrored disk write (cache is set > to none for the current disks)? Probably a question better asked on the qemu list, but yes, my understanding is that if you have a disk mirror or backup job set up, then qemu has to manage to flush I/O to two locations instead of one, and might end up slightly slowing the guest as a result.