Re: qcow2 overlay performance

[Yoonho Park]

Great. I will give your patches a try. Also, your workaround suggestion and a discussion with a colleague gave me another idea for an experiment. Is it possible that some of the overhead I am seeing is from the operations necessary to increase the size of the overlay? Would it make sense to use the workaround to create a "full-sized" overlay and then measure fio performance?

On Wed, Aug 26, 2020 at 10:57 AM Peter Krempa <pkrempa@redhat.com> wrote:

On Wed, Aug 26, 2020 at 15:30:03 +0200, Peter Krempa wrote:
> 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?

[...]

> 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.

After sending the previous mail I thought about it for a bit and IMO a
majority of cases would be fixed by using the cluster size of the
original image in the newly created image. I've created patches which do
this with libvirt. You can get them here:

https://www.redhat.com/archives/libvir-list/2020-August/msg00995.html