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

[Vladimir Sementsov-Ogievskiy]

17.06.2021 22:51, Vladimir Sementsov-Ogievskiy wrote:
> Recently we added support of zstd to qcow2 format, as zstd seems to be
> better than zlib in general, and which is important (as qcow2
> compression used mostly for backups) compressed writes are faster with
> zstd.
>
> Let's add a build option to use zstd by default.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>
> Hi all! We want to use zstd as a default compression type for newly
> created qcow2 images in upcoming Virtuozzo 8.
>
> I am not sure, how much the community interested in such option,
> probably I should just keep a downstream-only patch changing the
> default.
>
> But if you like it, I'd be happy to only set new config option in our
> qemu build instead of maintaining extra downstream-only patch :)
>
> 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?


First: the biggest difficulty with zstd as default is updating all iotests to 
work in such conditions. I'm working on this now and hope to post series soon, 
which just improves iotests to support IMGOPTS='compression_type=zstd'. That's 
a first step which is good anyway and don't change any default.

Next: I have a doubt now. Maybe, better way of having alternative default in 
downstream project is introcuducing a kind of configuration file for Qemu? Qemu 
will parse it if exist, and we can ship such a file in rpm package. Something 
like /etc/qemu.conf. Any thoughts? Did something like this ever considered?

Benefit of configuration file vs build options is obvious: you can change a 
default without recompiling.


--
Best regards,
Vladimir