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authorDenis V. Lunev <den@openvz.org>2016-06-02 18:58:15 +0300
committerKevin Wolf <kwolf@redhat.com>2016-06-08 10:21:09 +0200
commitf3c3b87dae44ac6c82246ceb3953793951800a9a (patch)
tree5e2ecaaf97b3fa9f224277c719464b7b62becc57 /block/qcow2-cache.c
parent6f6071745bd0366221f5a0160ed7d18d0e38b9f7 (diff)
qcow2: avoid extra flushes in qcow2
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>
Diffstat (limited to 'block/qcow2-cache.c')
-rw-r--r--block/qcow2-cache.c11
1 files changed, 9 insertions, 2 deletions
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 0fe8edae41..208a060421 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -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;
}