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Re: [PATCH v4 4/5] qcow2: add zstd cluster compression

From: Denis Plotnikov
Subject: Re: [PATCH v4 4/5] qcow2: add zstd cluster compression
Date: Wed, 4 Mar 2020 15:46:21 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 04.03.2020 10:49, Vladimir Sementsov-Ogievskiy wrote:
03.03.2020 16:34, 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>


+static ssize_t qcow2_zstd_compress(void *dest, size_t dest_size,
+                                   const void *src, size_t src_size)
+    size_t ret;
+    /*
+     * steal ZSTD_LEN_BUF bytes in the very beginning of the buffer
+     * to store compressed chunk size
+     */
+    char *d_buf = ((char *) dest) + ZSTD_LEN_BUF;
+    /*
+     * sanity check that we can store the compressed data length,
+     * and there is some space left for the compressor buffer
+     */
+    if (dest_size <= ZSTD_LEN_BUF) {
+        return -ENOMEM;
+    }
+    dest_size -= ZSTD_LEN_BUF;
+    ret = ZSTD_compress(d_buf, dest_size, src, src_size, 5);

You may want to define ZSTD_COMPRESSION_LEVEL constant instead of raw number.
I didn't introduce it intentionally. zlib compression has the compression level hardcoded as well. I think it's better to introduce the compression level for both of them in the future but not in the scope of this series.
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

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