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 <
address@hidden>
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 <
address@hidden>
---
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
---------------------------------------------------------------------------------------------------------------