[Qemu-block] [PATCH 1/2] ignore bdrv_flush operation when no qcow2 cache item is dirty

[Qingshu Chen]

qcow2_cache_flush() writes dirty cache to the disk and invokes bdrv_flush()
to make the data durable. But even if there is no dirty cache,
qcow2_cache_flush() would invoke bdrv_flush().  In fact, bdrv_flush() will
invoke fdatasync(), and it is an expensive operation. The patch will not
invoke bdrv_flush if there is not dirty cache. The reason that I modify the
return value of qcow2_cache_flush()  is qcow2_co_flush_to_os needs to know
whether flush operation is called. Following is the patch:

>From 23f9f83da4178e8fbb53d2cffe128f5a2d3a239a Mon Sep 17 00:00:00 2001
From: Qingshu Chen <qingshu.chen...@gmail.com>
Date: Wed, 1 Jul 2015 14:45:23 +0800
Subject: [PATCH 1/2] ignore bdrv_flush operation when no qcow2 cache item is
 dirty
Signed-off-by: Qingshu Chen <qingshu.chen...@gmail.com>

---
 block/qcow2-cache.c | 9 ++++++++-
 block/qcow2.c       | 2 ++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index ed92a09..57c0601 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -174,6 +174,7 @@ int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache
*c)
     int result = 0;
     int ret;
     int i;
+    int flag = 0;

     trace_qcow2_cache_flush(qemu_coroutine_self(), c == s->l2_table_cache);

@@ -182,15 +183,21 @@ int qcow2_cache_flush(BlockDriverState *bs,
Qcow2Cache *c)
         if (ret < 0 && result != -ENOSPC) {
             result = ret;
         }
+        if (c->entries[i].dirty && c->entries[i].offset) {
+            flag = 1;
+        }
     }

-    if (result == 0) {
+    if (result == 0 && flag == 1) {
         ret = bdrv_flush(bs->file);
         if (ret < 0) {
             result = ret;
         }
     }

+    if (flag == 0 && result >= 0) {
+        result = 1;
+    }
+
     return result;
 }

diff --git a/block/qcow2.c b/block/qcow2.c
index d522ec7..ab4613a 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2504,6 +2504,8 @@ static coroutine_fn int
qcow2_co_flush_to_os(BlockDriverState *bs)
     if (ret < 0) {
         qemu_co_mutex_unlock(&s->lock);
         return ret;
+    } else if (ret == 1) {
+        bdrv_flush(bs->file);
     }

     if (qcow2_need_accurate_refcounts(s)) {
--
1.9.1





-------------------------------------------------------------------------------------------------------------
Qingshu Chen,
Institute of Parallel and Distributed Systems (IPADS),
School of Software,
Shanghai Jiao Tong University
---------------------------------------------------------------------------------------------------------------