qcow2 decompress in threads

Hi all!

The series brings threads to qcow2 decompression path, like it is already done for compression.

Performance gain is illustrated by the following test:

[]# cat gen.sh
#!/bin/bash

echo 'create pattern-file /ssd/t_pat'

./qemu-img create -f raw /ssd/t_pat 10g
for i in {0..9}; do

    ./qemu-io -f raw -c "write -P 0xab ${i}g 1g" /ssd/t_pat

done

echo 'convert it to compressed /ssd/t_pat.compressed.qcow2'

./qemu-img convert -W -f raw -O qcow2 -c /ssd/t_pat /ssd/t_pat.compressed.qcow2 rm -f /ssd/t_pat

test:
[]# time ./qemu-img convert -f qcow2 --target-image-opts -n /ssd/t_pat.compressed.qcow2 'driver=null-co,size=10G'

result before the series:

real    0m16.585s
user    0m14.899s
sys     0m2.219s

result after the series:

real    0m6.528s
user    0m19.343s
sys     0m3.081s

Note: my cpu is 4-cores 8-threads i7-4790

Vladimir Sementsov-Ogievskiy (7):
qcow2: use Z_OK instead of 0 for deflateInit2 return code check
qcow2: make more generic interface for qcow2_compress
qcow2: move decompression from qcow2-cluster.c to qcow2.c
qcow2: refactor decompress_buffer
qcow2: use byte-based read in qcow2_decompress_cluster
qcow2: aio support for compressed cluster read
qcow2: do decompression in threads

 block/qcow2.h         |   4 -
 block/qcow2-cluster.c |  70 -----------------
 block/qcow2.c         | 169 +++++++++++++++++++++++++++++++++++-------
 3 files changed, 143 insertions(+), 100 deletions(-)

2.18.0