Re: [Qemu-devel] [PATCH v0 3/3] qcow2: add zstd cluster compression

[Kevin Wolf <kwolf@redhat.com>]

Am 02.07.2019 um 14:49 hat Denis Plotnikov geschrieben:
> On 28.06.2019 14:57, Kevin Wolf wrote:
> > Am 28.05.2019 um 16:37 hat Denis Plotnikov geschrieben:
> >> diff --git a/configure b/configure
> >> index 1c563a7027..c90716189c 100755
> >> --- a/configure
> >> +++ b/configure
> >> @@ -433,6 +433,7 @@ opengl_dmabuf="no"
> >>   cpuid_h="no"
> >>   avx2_opt=""
> >>   zlib="yes"
> >> +zstd="yes"
> > 
> > This should be zstd="" so that a missing library will automatically
> > disable it instead of producing an error. (Building QEMU without zlib is
> > impossible, but building it without ZSTD should work.)
> But if we add zstd for clusters compression we have to build with zstd 
> each time. If we want to chose whether we want to build zstd we need to
> enclose all zstd related code with "ifdef"-s.
> I don't think it's good because we can end up with mess of version 
> supporting and not supporting zstd compression.

Yes, we'll need ifdefs. Or we could do it like the dmg compression
formats, a spearate source file for zstd that is compiled conditionally.

Anyway, I don't think making zstd a hard dependency for qemu is
acceptable.

> Another point is what the benefit of building qemu without zstd support 
> is since it's available and provides better performance than zlib (i.e. 
> the replacement for zlib) ?

Is it available in all relevant distros and even non-Linux platforms
that we support?

Another reason for wanting to compile it out even when it is available
is startup time. Each shared library to be loaded takes some time, and
there are use cases where you want a minimal guest to boot up really
fast.

Kevin