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

[Vladimir Sementsov-Ogievskiy]

21.06.2021 18:59, Kevin Wolf wrote:
> 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.

Looks good, I'll try. I'm just not familiar with it and followed the pattern of 
zstd option.

> > 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.

I'm afraid it does anyway: incompatible feature bit is in use for the feature. 
Older Qemu will refuse opening image with incompatible bit

> 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.

More details:

- compression type is specified once, when image is created by qemu-img create 
Or blockdev-create. At that time incompatible bit is set if zstd is chosen.

- compressed writes are done by
  - qemu-img convert -c
  - backup jobs (blockdev-backup adn drive-backup) when option compress is set 
to true
  - compress filter (which is available to insert where user wants)

- drive-backup may create target image automatically or use existing image 
depending on options.

I have a plan to deprecate drive-backup (patches are somewhere in the list and 
waits for possibility to deprecate union fields in QAPI).

I think preferred way of compression would be compress-filter, and we may want 
to deprecate compress option of blockdev-backup at some moment.


--
Best regards,
Vladimir