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From a1a7f1301c20582d3326e33acd94a4c198c60bcd Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 18 Oct 2013 08:14:27 +0200
Subject: [PATCH 02/81] cow: make writes go at a less indecent speed

RH-Author: Paolo Bonzini <pbonzini@redhat.com>
Message-id: <1382084091-16636-3-git-send-email-pbonzini@redhat.com>
Patchwork-id: 54985
O-Subject: [RHEL 7.0 qemu-kvm PATCH 02/26] cow: make writes go at a less indecent speed
Bugzilla: 989646
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>

Only sync once per write, rather than once per sector.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 26ae980492920e417bc91761cc85950f18e41f85)
---
 block/cow.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/cow.c |   19 ++++++++++++++++---
 1 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/block/cow.c b/block/cow.c
index c12088b..2f72334 100644
--- a/block/cow.c
+++ b/block/cow.c
@@ -106,7 +106,7 @@ static int cow_open(BlockDriverState *bs, QDict *options, int flags)
  * XXX(hch): right now these functions are extremely inefficient.
  * We should just read the whole bitmap we'll need in one go instead.
  */
-static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
+static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum, bool *first)
 {
     uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
     uint8_t bitmap;
@@ -117,9 +117,21 @@ static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
        return ret;
     }
 
+    if (bitmap & (1 << (bitnum % 8))) {
+        return 0;
+    }
+
+    if (*first) {
+        ret = bdrv_flush(bs->file);
+        if (ret < 0) {
+            return ret;
+        }
+        *first = false;
+    }
+
     bitmap |= (1 << (bitnum % 8));
 
-    ret = bdrv_pwrite_sync(bs->file, offset, &bitmap, sizeof(bitmap));
+    ret = bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap));
     if (ret < 0) {
        return ret;
     }
@@ -181,9 +193,10 @@ static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
 {
     int error = 0;
     int i;
+    bool first = true;
 
     for (i = 0; i < nb_sectors; i++) {
-        error = cow_set_bit(bs, sector_num + i);
+        error = cow_set_bit(bs, sector_num + i, &first);
         if (error) {
             break;
         }
-- 
1.7.1