[PATCH 1/3] Add public API for parallel compression method

[Jiri Denemark]

On Tue, Jan 17, 2023 at 21:58:39 +0800, Jiang Jiacheng wrote:
> 
> 
> On 2023/1/17 16:44, Claudio Fontana wrote:
> > Hi,
> > 
> > On 1/16/23 14:42, Jiang Jiacheng wrote:
> >> Add public API VIR_MIGRATE_PARAM_PARALLEL_COMPRESSION,
> >> VIR_MIGRATE_PARAM_PARALLEL_ZLIB_LEVEL, VIR_MIGRATE_PARAM_PARALLEL_ZSTD_LEVEL
> >> for migration APIs to support set compression method
> >> and compress level used during migration.
> >>
> >> Signed-off-by: Jiang Jiacheng <jiangjiacheng at huawei.com>
> > 
> > we don't know what drivers other than QEMU are going to implement in terms of algorithms,
> > and we also don't know what new algorithms QEMU will come up with in the future,
> > 
> 
> Indeed, what you're talking about is better and more scalable. These
> patches only adapt to the three parameters of the current version
> QEMU，and some commits on the QEMU side may be required to support this
> mode.
> 
> > so I was told when I was working on something similar (but not identical),
> > for the "multifd save restore prototype" to try to make it more general.
> > 
> > First of all, maybe we don't need two different _LEVEL parameters,
> > just a VIR_MIGRATE_PARAM_PARALLEL_COMPRESSION_LEVEL, which is then mapped in the qemu driver to the specific correct QEMU parameter according to the chosen algo.
> > 
> 
> Since QEMU sets different parameters for different compression methods,
> I've added two corresponding parameters here. If the qemu side does not
> change, we can use the same virsh option to input the compression level
> and pass the expected QEMU parameters according to the input compression
> method on the libvirt side.
> 
> > Then the VIR_MIGRATE_PARAM_PARALLEL_COMPRESSION could be a freeform string, that is then interpreted by each driver, for example "zlib", or "zstd" in this case.
> > So other drivers can pass different strings, and QEMU future new compression algorithms will not need changes to libvirt at all.
> 
> I think what you mean is we can input any string to
> VIR_MIGRATE_PARAM_PARALLEL_COMPRESSION and whether it is available is
> judged by QEMU or other emulators.
> It is supported with these patches, we can use any string for this
> parameter, though QEMU only support 'none', 'zlib','zstd' for it, and
> other input will return with an error.

I think we should just be able to use the existing
VIR_MIGRATE_PARAM_COMPRESSION to specify the method. We know whether
we're asking for parallel migration from VIR_MIGRATE_PARALLEL flag. We
might need to add checks for which compression methods are only allowed
with/without VIR_MIGRATE_PARALLEL.

On the other hand, for consistency reasons I think we should add
separate VIR_MIGRATE_PARAM_COMPRESSION_ZLIB_LEVEL and
VIR_MIGRATE_PARAM_COMPRESSION_ZSTD_LEVEL even though they are the same.
The other two compressions methods we currently support (mt and
xbzrle) have each their own set of parameters.

Jirka