[dm-devel] direct-io regression [Was: How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs]

[Chris Mason]

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 at redhat.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).

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