Re: [PATCH v22 3/4] qcow2: add zstd cluster compression

[Vladimir Sementsov-Ogievskiy]

29.04.2020 13:24, Max Reitz wrote:
> On 28.04.20 22:00, Denis Plotnikov wrote:
> > zstd significantly reduces cluster compression time.
> > It provides better compression performance maintaining
> > the same level of the compression ratio in comparison with
> > zlib, which, at the moment, is the only compression
> > method available.
> >
> > The performance test results:
> > Test compresses and decompresses qemu qcow2 image with just
> > installed rhel-7.6 guest.
> > Image cluster size: 64K. Image on disk size: 2.2G
> >
> > The test was conducted with brd disk to reduce the influence
> > of disk subsystem to the test results.
> > The results is given in seconds.
> >
> > compress cmd:
> >    time ./qemu-img convert -O qcow2 -c -o compression_type=[zlib|zstd]
> >                    src.img [zlib|zstd]_compressed.img
> > decompress cmd
> >    time ./qemu-img convert -O qcow2
> >                    [zlib|zstd]_compressed.img uncompressed.img
> >
> >             compression               decompression
> >           zlib       zstd           zlib         zstd
> > ------------------------------------------------------------
> > real     65.5       16.3 (-75 %)    1.9          1.6 (-16 %)
> > user     65.0       15.8            5.3          2.5
> > sys       3.3        0.2            2.0          2.0
> >
> > Both ZLIB and ZSTD gave the same compression ratio: 1.57
> > compressed image size in both cases: 1.4G
> >
> > Signed-off-by: Denis Plotnikov <address@hidden>
> > QAPI part:
> > Acked-by: Markus Armbruster <address@hidden>
> > ---
> >   docs/interop/qcow2.txt |   1 +
> >   configure              |   2 +-
> >   qapi/block-core.json   |   3 +-
> >   block/qcow2-threads.c  | 169 +++++++++++++++++++++++++++++++++++++++++
> >   block/qcow2.c          |   7 ++
> >   slirp                  |   2 +-
> >   6 files changed, 181 insertions(+), 3 deletions(-)
>
> [...]
>
> > diff --git a/block/qcow2-threads.c b/block/qcow2-threads.c
> > index 7dbaf53489..a0b12e1b15 100644
> > --- a/block/qcow2-threads.c
> > +++ b/block/qcow2-threads.c
>
> [...]
>
> > +static ssize_t qcow2_zstd_decompress(void *dest, size_t dest_size,
> > +                                     const void *src, size_t src_size)
> > +{
>
> [...]
>
> > +    /*
> > +     * The compressed stream from the input buffer may consist of more
> > +     * than one zstd frame.
>
> Can it?

If not, we must require it in the specification. Hmm. If at some point we'll 
want multi-threaded compression of one big (2M) cluster.. Could this be 
implemented with zstd lib, if multiple frames are allowed, will allowing 
multiple frames help? I don't know actually, but I think better not to forbid 
it. On the other hand, I don't see any benefit in large compressed clusters. At 
least, in our scenarios (for compressed backups) we use 64k compressed 
clusters, for good granularity of incremental backups (when for running vm we 
use 1M clusters).



--
Best regards,
Vladimir