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Re: direct-io regression [Was: How to track down abysmal performance ata

To: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>, Valdis.Kletnieks@xxxxxx, josef@xxxxxxxxxx, Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, dm-devel@xxxxxxxxxx
Subject: Re: direct-io regression [Was: How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs]
From: Chris Mason <chris.mason@xxxxxxxxxx>
Date: Thu, 5 Aug 2010 07:32:40 -0400
In-reply-to: <20100805093100.GA3001@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Mail-followup-to: Chris Mason <chris.mason@xxxxxxxxxx>, Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>, Valdis.Kletnieks@xxxxxx, josef@xxxxxxxxxx, Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, dm-devel@xxxxxxxxxx
References: <20100804073546.GA7494@xxxxxxxxxxxxxxxxxxxxxxxxxx> <20100804085039.GA11671@xxxxxxxxxxxxx> <20100804091317.GA27779@xxxxxxxxxxxxxxxxxx> <20100804092122.GA2998@xxxxxxxxxxxxx> <20100804073546.GA7494@xxxxxxxxxxxxxxxxxxxxxxxxxx> <201008041116.09822@xxxxxx> <20100804102526.GB13766@xxxxxxxxxxxxxxxxxx> <15446.1280953986@localhost> <20100805093100.GA3001@xxxxxxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Thu, Aug 05, 2010 at 11:31:00AM +0200, Dominik Brodowski wrote:
> 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@xxxxxxxxxx>
> 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).

Well, you've already bisected down to an offending if statement, that's
a huge help.  I'll try to reproduce this and fix it up today.

But, I'm surprised your drive is doing 8K dio reads at 16MB/s, that
seems a little high.  

-chris

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