Re: [Qemu-devel] [PATCH v2 3/3] qcow2: add zstd cluster compression

[Denis Plotnikov]

On Jul 9 2019, at 9:18 am, Markus Armbruster <address@hidden> wrote:
Denis Plotnikov <address@hidden> writes:

zstd significantly reduces cluster compression time.
It provides better compression performance maintaining
the same level of compression ratio in comparison with
zlib, which, by the moment, has been 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>
[...]
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 7cf068f814..4344e858cb 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -538,6 +538,9 @@ Compressed Clusters Descriptor (x = 62 - (cluster_bits - 
8)):
Another compressed cluster may map to the tail of the final
sector used by this compressed cluster.

+ The layout of the compressed data depends on the compression
+ type used for the image (see compressed cluster layout).
+
If a cluster is unallocated, read requests shall read the data from the backing
file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
no backing file or the backing file is smaller than the image, they shall read
@@ -790,3 +793,19 @@ In the image file the 'enabled' state is reflected by the 
'auto' flag. If this
flag is set, the software must consider the bitmap as 'enabled' and start
tracking virtual disk changes to this bitmap from the first write to the
virtual disk. If this flag is not set then the bitmap is disabled.
+
+=== Compressed cluster layout ===
+
+The compressed cluster data may have a different layout depending on the
+compression type used for the image, and store specific data for the particular
+compression type.
+
+Compressed data layout for the available compression types:
+(x = data_space_length - 1)
+
+ zlib:
+ Byte 0 - x: the compressed data content
+ all the space provided used for compressed data
+ zstd:
+ Byte 0 - 3: the length of compressed data
+ 4 - x: the compressed data content

Adding <http://zlib.net/> and <http://github.com/facebook/zstd> here as
well wouldn't hurt, would it?
ok

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 835dd3c37f..2021e03a84 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -4215,11 +4215,12 @@
# Compression type used in qcow2 image file
#
# @zlib: zlib compression, see <http://zlib.net/>
+# @zstd: zstd compression, see <http://github.com/facebook/zstd>
#
# Since: 4.1
##
{ 'enum': 'Qcow2CompressionType',
- 'data': [ 'zlib' ] }
+ 'data': [ 'zlib', { 'name': 'zstd', 'if': 'defined(CONFIG_ZSTD)' } ] }

##
# @BlockdevCreateOptionsQcow2:

QAPI schema
Acked-by: Markus Armbruster <address@hidden>