[Qemu-devel] [patch 0/5][v3] Improve qcow2 performance when used with cache=off.

[Laurent . Vivier]

These patches improve qcow2 performance when used with cache=off.

They modify block-qcow2.c to read/write as many clusters as
possible per bdrv_aio_[read|write]().

These patches have been tested several hours, with normal disk, encrypted disk,
compressed disk and with backing file or not.
Tests were done with mkfs, untar of kernel source, build of kernel 
sources, rm of all files, dbench 1, dbench 16 and dd to fill the disk (500 MB).
fsck is used every time to check filesystem consistency.

Some benchmarks (from [v2]):

* mkfs on 500 MB qcow2 disk

mkfs                            WITHOUT         WITH

ide, cache=off,snapshot=off     41 s            5 s     8x faster
ide, cache=off,snapshot=on      40 s            5 s     8x faster
ide, cache=on, snapshot=off      3 s            3 s
ide, cache=on, snapshot=on       4 s            3 s

* tar jxvf linux-2.6.25.7.tar.bz2

ide, cache=off,snapshot=off     847 s           379 s   2.2x faster
ide, cache=off,snapshot=on      801 s           364 s   2.2x faster
ide, cache=on, snapshot=off     238 s           236 s
ide, cache=on, snapshot=on      236 s           237 s

* dd if=/dev/zero of=file

dd if=/dev/null                 WITHOUT         WITH

ide, cache=off,snapshot=off     333 kB/s        3.7 MB/s  11.3x faster
ide, cache=off,snapshot=on      337 kB/s        3.6 MB/s  10.9x faster
ide, cache=on, snapshot=off    9.06 MB/s        9.23 MB/s
ide, cache=on, snapshot=on     8,89 MB/s        8.89 MB/s

* dbench 1

dbench                          WITHOUT          WITH

ide, cache=off,snapshot=off     20.8494 MB/sec  24.8521 MB/sec          +19,2%
ide, cache=off,snapshot=on      20.9349 MB/sec  24.2296 MB/sec          +15,7%
ide, cache=on, snapshot=off     23.6612 MB/sec  25.4724 MB/sec          + 7,6%
ide, cache=on, snapshot=on      24.1836 MB/sec  24.8169 MB/sec

Changelog: 
v2: follow the advice of Avi kivity, and modify get_cluster_offset()
v3: follow comments from Kevin Wolf, and add some comments, remove
    free_used_clusters().
    follow comments from Anthony Liguori, and make more tests,
    correct a SEGV with encrypted disk (too small buffer),
    and a file corruption on commit.

[PATCH 1/5] extract code from get_cluster_offset() into new functions
            seek_l2_table(), l2_load() and l2_allocate().

[PATCH 2/5] divide get_cluster_offset() into get_cluster_offset() and
            alloc_cluster_offset().

[PATCH 3/5] divide alloc_cluster_offset() into alloc_cluster_offset()
            and alloc_compressed_cluster_offset().

[PATCH 4/5] modify get_cluster_offset(), alloc_cluster_offset() to specify 
            how many clusters we want.

[PATCH 5/5] in alloc_cluster_offset(), try to aggregate free clusters
            and freed clusters.
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