Re: [PATCH RFC] meson: add option to use zstd for qcow2 compression by default

[Kevin Wolf]

Am 21.06.2021 um 10:22 hat Paolo Bonzini geschrieben:
> On 17/06/21 21:51, Vladimir Sementsov-Ogievskiy wrote:
> > So, it's an RFC. I also can split the patch so that refactoring of
> > qcow2_co_create() go in a separate preparation patch.
> > 
> > Another RFC question, shouldn't we move to zstd by default in upstream
> > too?
> 
> I think backwards-incompatible changes in the past were not handled with
> build options, but that can be changed.
> 
> However I would prefer to have an option like
> --with-qcow2-compression={zstd,zlib}.  Meson supports multiple-choice
> options, they don't have to use enabled/disabled or (if boolean) true/false.

Yes, this is more extensible.

> Regarding changing the default, that would make images unreadable to QEMU
> 5.0 and earlier versions.  Does that apply to images that have no compressed
> clusters?

I think it does because you could be writing compressed clusters to it
later. Originally, we had only 'qemu-img convert -c' that could write
compressed clusters, but today the backup job can write them, too, and
it doesn't create the image file itself.

Kevin