[Question] Which qcow2 options could be inherited to snapshot/blockcopy?

[Meina Li]

Hi Peter,

I can learn a lot from what you said. Thank you very much.



Best Regards,
Meina Li


On Mon, Nov 21, 2022 at 5:30 PM Peter Krempa <pkrempa at redhat.com> wrote:

> On Mon, Nov 21, 2022 at 11:22:40 +0800, Meina Li wrote:
> > Hi,
> >
> > I'm trying to find out which qcow2 options could be inherited to
> > snapshot/blockcopy and then test them.
> >
> > From https://github.com/qemu/qemu/blob/master/qapi/block-core.json#L4720
> we
> > can know all the qcow2 options. As far as I know, size, cluster_size and
> > extended-l2 have already been implemented. So I'm curious that:
> > 1) What are the current status and future plans of other options? Like
> > compat option.
>
> The 'size' option is needed obviously to have a correctly sized image.
>
> With 'cluster_size' and 'extended_l2' those options were identified as
> having potential performance implications and users actually wanting to
> tweak them for their images. Thus we inherit them.
>
> For anything else there wasn't any specific request or noting that it
> can have performance implications so they are omitted for now.
>
> In regards to the 'compat' option in terms of snapshots/blockcopy, we
> don't set it and thus use the qemu default. Since both operations create
> a new image with an existing qemu instance, the default new qemu format
> is okay.
>
> For the other options:
>
>  - 'encrypt' and co.
>     - encryption can be explicitly enabled via XML
>  - 'data-file-raw'
>     - not supported by libvirt, no plans for now
>  - 'preallocation'
>     - not implemented
>     - some options don't make sense, e.g. full allocation for a snapshot
>  - 'lazy-refcounts'/'refcount-bits'
>     - not implemented, no plans
>  - 'compression-type'
>     - libvirt for now doesn't allow to use compression IIRC
>
> > 2) Also whether the vol-clone can cover all options?
>
> Note that 'vol-clone' uses qemu-img, so the logic is different there.
>
> > Thank you very much. And please help to correct me if I have something
> > wrong.
> >
> > Best Regards,
> > Meina Li
>
>
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