linux-ext4 - A doubt on journal_async

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Date:	Fri, 12 Dec 2014 14:03:32 +0800
From:	alex chen <alex.chen@...wei.com>
To:	<tytso@....edu>, Jan Kara <jack@...e.cz>,
	<linux-ext4@...r.kernel.org>
CC:	Joseph Qi <joseph.qi@...wei.com>
Subject: A doubt on journal_async_commit option

This commit 0e3d2a6313(ext4: Fix async commit mode to be safe by using
a barrier) show that using journal_async_commit feature has a 50%
performance improvement. But I tested in SUSE Linux Enterprise Server
11 SP3(linux kernel 3.0.93) and Red Hat Enterprise linux 6.4(linux
kernel 2.6.32), the result show this feature has no performance
improvement.
My test command:
mount /dev/sdb /mnt/sdb
./fs_mark  -d  /mnt/sdb/  -s  10240  -n  1000
umount

mount /dev/sdb /mnt/sdb -o journal_async_commit
./fs_mark  -d  /mnt/sdb/  -s  10240  -n  1000
umount

My test result:
FSUse%        Count         Size    Files/sec     App Overhead
     6         1000        10240         42.1            10671
vs.
-o journal_async_commit
FSUse%        Count         Size    Files/sec     App Overhead
     6         1000        10240         63.9            10625

Superblock info(-o journal_async_commit):
Filesystem features:      has_journal ext_attr resize_inode dir_index
filetype needs_recovery extent flex_bg sparse_super large_file
huge_file
uninit_bg dir_nlink extra_isize
Block size:               4096
Journal backup:           inode blocks
Journal features:         journal_checksum journal_async_commit
Journal size:             128M
Journal length:           32768
Journal sequence:         0x000017a6
Journal start:            1

Am I missing something?

                                            ---Alex

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