File 0074-qcow2-avoid-extra-flushes-in-qcow2.patch of Package qemu

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File 0074-qcow2-avoid-extra-flushes-in-qcow2.patch of Package qemu (Revision 493cc9aa96d045bf944d0959c722d379)

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From 197d526012602fbac75392a86e991539e4400bf0 Mon Sep 17 00:00:00 2001
From: "Denis V. Lunev" <den@openvz.org>
Date: Thu, 2 Jun 2016 18:58:15 +0300
Subject: [PATCH] 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>
(cherry picked from commit f3c3b87dae44ac6c82246ceb3953793951800a9a)
[BR: BSC#991296]
Signed-off-by: Bruce Rogers <brogers@suse.com>
---
 block/qcow2-cache.c | 11 +++++++++--
 block/qcow2.c       |  4 ++--
 block/qcow2.h       |  1 +
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 0fe8eda..208a060 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;
         }
diff --git a/block/qcow2.c b/block/qcow2.c
index 470734b..dc609a1 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2774,14 +2774,14 @@ 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;
diff --git a/block/qcow2.h b/block/qcow2.h
index a063a3c..7db9795 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -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);

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File 0074-qcow2-avoid-extra-flushes-in-qcow2.patch of Package qemu (Revision 493cc9aa96d045bf944d0959c722d379)

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