[dm-devel] fragmented i/o with 2.6.31?

[David Strand]

I should note that I am using dm-mpath, and the i/o is fragmented on
the wire when using the device mapper device node but it is not
fragmented when using one of the regular /dev/sd* device nodes for
that device.

On Wed, Sep 16, 2009 at 8:34 AM, David Strand <dpstrand at gmail.com> wrote:
> I am issuing 512 Kbyte reads through the device mapper device node to
> a fibre channel disk. With 2.6.30 one read command for the entire 512
> Kbyte length is placed on the wire. With 2.6.31 this is being broken
> up into 5 smaller read commands placed on the wire, decreasing
> performance.
>
> This is especially penalizing on some disks where we have prefetch
> turned off via the scsi mode page. Is there any easy way (through
> configuration or sysfs) to restore the single read per i/o behavior
> that I used to get?
>