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

[Qingshu Chen <qingshu.chen714@gmail.com>]

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I've made a mistake on the series. Following is the new patch:

>From ef1079b422eef40a802ca13e249795005efa441d Mon Sep 17 00:00:00 2001
From: Qingshu Chen <1150163259@qq.com>
Date: Tue, 21 Jul 2015 09:46:08 +0800
Subject: [PATCH] ignore bdrv_flush operation when no qcow2 cache item is
dirty
Signed-off-by: Qingshu Chen <qingshu.chen714@gmail.com>

---
 block/qcow2-cache.c | 16 ++++++++++++----
 block/qcow2.c       |  2 ++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 53b8afc..08d1884 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -174,23 +174,31 @@ int qcow2_cache_flush(BlockDriverState *bs,
Qcow2Cache *c)
     int result = 0;
     int ret;
     int i;
+    bool flag = false;

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

     for (i = 0; i < c->size; i++) {
-        ret = qcow2_cache_entry_flush(bs, c, i);
-        if (ret < 0 && result != -ENOSPC) {
-            result = ret;
+        if (c->entries[i].dirty && c->entries[i].offset) {
+            flag = true;
+            ret = qcow2_cache_entry_flush(bs, c, i);
+            if (ret < 0 && result != -ENOSPC) {
+                result = ret;
+            }
         }
     }

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

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

diff --git a/block/qcow2.c b/block/qcow2.c
index 76c331b..2e95cc1 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2505,6 +2505,8 @@ 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


2015-07-20 23:03 GMT+08:00 Eric Blake <eblake@redhat.com>:
>
> [patches should always be sent to qemu-devel, even if qemu-block is also
> in the to/cc list]
>
> On 07/08/2015 01:26 AM, Qingshu Chen wrote:
> > 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.chen714@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.chen714@gmail.com>
>
> I didn't quickly find an associated 2/2 patch; are you sure you sent the
> series correctly?
>
> >
> > ---
> >  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;
>
> This is used as a bool, so declare it as such (bool flag = false;).
>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>



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

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