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

[Max Reitz <mreitz@redhat.com>]

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On 29.04.20 12:38, Denis Plotnikov wrote:
> 
> 
> On 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 <dplotnikov@virtuozzo.com>
>>> QAPI part:
>>> Acked-by: Markus Armbruster <armbru@redhat.com>
>>> ---
>>>   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?
> 
> Potentially, it can, if another implemention of qcow2 saves a couple of
> frames for some reason.

Well, it can’t do that if qemu cannot decode it.  Maybe there should be
a note in the specification on the precise format to expect.

If we decide that it might make sense for someone to encode a cluster
with multiple frames, then OK.

Max


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