On 30.04.2020 11:26, Max Reitz wrote:
On 29.04.20 15:02, Vladimir Sementsov-Ogievskiy wrote:
29.04.2020 15:17, Max Reitz wrote:
On 29.04.20 12:37, Vladimir Sementsov-Ogievskiy wrote:
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 <dplotni...@virtuozzo.com>
QAPI part:
Acked-by: Markus Armbruster <arm...@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?
If not, we must require it in the specification.
Actually, now that you mention it, it would make sense anyway to add
some note to the specification on what exactly compressed with zstd
means.
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).
Is it really that important? Naïvely, it sounds rather complicated to
introduce multithreading into block drivers.
It is already here: compression and encryption already multithreaded.
But of course, one cluster is handled in one thread.
Ah, good. I forgot.
(Also, as for compression, it can only be used in backup scenarios
anyway, where you write many clusters at once. So parallelism on the
cluster level should sufficient to get high usage, and it would benefit
all compression types and cluster sizes.)
Yes it works in this way already :)
Well, OK then.
So, we don't know do we want one frame restriction or not. Do you have a
preference?
*shrug*
Seems like it would be preferential to allow multiple frames still. A
note in the spec would be nice (i.e., streaming format, multiple frames
per cluster possible).
We don't mention anything about zlib compressing details in the spec.
If we mention anything about ZSTD compressing details we'll have to do
it for
zlib as well. So, I think we have two possibilities for the spec:
1. mention for both
2. don't mention at all
I think the 2nd is better. It gives more degree of freedom for the
future improvements.
and we've already covered both cases (one frame, may frames) in the code.
I'm note sure I'm right. Any other opinions?
Denis
Max
