Re: qcow2 overlay performance

[Peter Krempa]

On Wed, Aug 26, 2020 at 15:18:32 +0200, Kevin Wolf wrote:
> Am 26.08.2020 um 02:46 hat Yoonho Park geschrieben:
> > Another issue I hit is that I cannot set or change the cluster size of
> > overlays. Is this possible with "virsh snapshot-create-as"?
> 
> That's a libvirt question. Peter, can you help?

Currently the libvirt snapshot API doesn't support configuration of the
cluster size of the new image, (it should be straightforward to
implement it now that we create images via blockdev-create, so the only
open question is how to expose it in the XML).

There's a reasonalby straightforward workaround (at least for testing),
depending on how you want to approach it. You can create your own
overlay file using qemu-img create (make sure to properly specify
-F and -b) using your desired cluster size. You then use 'virsh
snapshot-create --reuse-external' which expects that the images
described by the snapshot XML are already existing files and uses those
(without touching the metadata at all) rather than creating new files.

If you figure out that cluster size matters a lot and qemu will not want
to change the default, please file a feature request for libvirt.

Either way, the best performing option should be the default though.