linux-kernel - direct-io regression [Was: How to track down abysmal performance ata - raid1 - crypto - vg/lv

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Message-ID: <20100805093100.GA3001@comet.dominikbrodowski.net>
Date:	Thu, 5 Aug 2010 11:31:00 +0200
From:	Dominik Brodowski <linux@...inikbrodowski.net>
To:	Valdis.Kletnieks@...edu, josef@...hat.com, chris.mason@...cle.com
Cc:	Michael Monnerie <michael.monnerie@...it-management.at>,
	Christoph Hellwig <hch@...radead.org>,
	linux-raid@...r.kernel.org, xfs@....sgi.com,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: direct-io regression [Was: How to track down abysmal performance
 ata - raid1 - crypto - vg/lv - xfs]

Hey,

when attempting to track down insufficient I/O performance, I found the
following reression relating to direct-io on my notebook, where an
ata device, which consists of several partitions, is combined to a lvm
volume, and one logical volume is then encrypted using dm-crypt. Test case
was the following command:

$ dd if=/dev/mapper/vg0-root_crypt of=/dev/zero iflag=direct bs=8k count=131072

2.6.34 results in ~16 MB/s,
2.6.35 results in ~ 3.1 MB/s

The regression was bisected down to the follwoing commit:

commit c2c6ca417e2db7a519e6e92c82f4a933d940d076
Author: Josef Bacik <josef@...hat.com>
Date:   Sun May 23 11:00:55 2010 -0400

    direct-io: do not merge logically non-contiguous requests

...

How to fix this? I do not use btrfs, but ext3 (and the access was down on
the block level, not on the fs level, so this btrs-related commit should not
cause such a regression).

Best,

	Dominik
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