From c6adc9cc082e3cffda153999c9b9f8a8baaaaf45 Mon Sep 17 00:00:00 2001
From: Miao Xie <miaox@cn.fujitsu.com>
Date: Tue, 28 May 2013 10:05:39 +0000
Subject: [PATCH] Btrfs: merge pending IO for tree log write back

Before applying this patch, we flushed the log tree of the fs/file
tree firstly, and then flushed the log root tree. It is ineffective,
especially on the hard disk. This patch improved this problem by wrapping
the above two flushes by the same blk_plug.

By test, the performance of the sync write went up ~60%(2.9MB/s -> 4.6MB/s)
on my scsi disk whose disk buffer was enabled.

Test step:
 # mkfs.btrfs -f -m single <disk>
 # mount <disk> <mnt>
 # dd if=/dev/zero of=<mnt>/file0 bs=32K count=1024 oflag=sync

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
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