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qcow2: avoid extra flushes in qcow2

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The problem with excessive flushing was found by a couple of performance
tests:
  - parallel directory tree creation (from 2 processes)
  - 32 cached writes + fsync at the end in a loop

For the first one results improved from 2.6 loops/sec to 3.5 loops/sec.
Each loop creates 10^3 directories with 10 files in each.

For the second one results improved from ~600 fsync/sec to ~1100
fsync/sec. Though, it was run on SSD so it probably won't show such
performance gain on rotational media.

qcow2_cache_flush() calls bdrv_flush() unconditionally after writing
cache entries of a particular cache. This can lead to as many as
2 additional fdatasyncs inside bdrv_flush.

We can simply skip all fdatasync calls inside qcow2_co_flush_to_os
as bdrv_flush for sure will do the job. These flushes are necessary to
keep the right order of writes to the different caches. Though this is
not necessary in the current code base as this ordering is ensured through
the flush in qcow2_cache_flush_dependency().

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Pavel Borzenkov <pborzenkov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Denis V. Lunev 2016-06-02 18:58:15 +03:00 committed by Kevin Wolf
parent 6f6071745b
commit f3c3b87dae
3 changed files with 12 additions and 4 deletions
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11
block/qcow2-cache.c
View file

@ -226,7 +226,7 @@ static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
return 0;
}
int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
int qcow2_cache_write(BlockDriverState *bs, Qcow2Cache *c)
{
BDRVQcow2State *s = bs->opaque;
int result = 0;
@ -242,8 +242,15 @@ int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
}
}
return result;
}
int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
{
int result = qcow2_cache_write(bs, c);
if (result == 0) {
ret = bdrv_flush(bs->file->bs);
int ret = bdrv_flush(bs->file->bs);
if (ret < 0) {
result = ret;
}

4
block/qcow2.c
View file

@ -2813,14 +2813,14 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs)
int ret;
qemu_co_mutex_lock(&s->lock);
ret = qcow2_cache_flush(bs, s->l2_table_cache);
ret = qcow2_cache_write(bs, s->l2_table_cache);
if (ret < 0) {
qemu_co_mutex_unlock(&s->lock);
return ret;
}
if (qcow2_need_accurate_refcounts(s)) {
ret = qcow2_cache_flush(bs, s->refcount_block_cache);
ret = qcow2_cache_write(bs, s->refcount_block_cache);
if (ret < 0) {
qemu_co_mutex_unlock(&s->lock);
return ret;

1
block/qcow2.h
View file

@ -583,6 +583,7 @@ int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c);
void qcow2_cache_entry_mark_dirty(BlockDriverState *bs, Qcow2Cache *c,
void *table);
int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
int qcow2_cache_write(BlockDriverState *bs, Qcow2Cache *c);
int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
Qcow2Cache *dependency);
void qcow2_cache_depends_on_flush(Qcow2Cache *c);