Jeff Moyer <jmoyer@xxxxxxxxxx> writes:
> Chuck Tuffli <ctuffli@xxxxxxxxx> writes:
>
>> On Thu, Mar 29, 2012 at 10:14 AM, Chris Worley <worleys@xxxxxxxxx> wrote:
>>> Sequential merging? Try "randread" instead.
>>
>> Great suggestion. Doing some additional analysis, it does appear that
>> IO's are probably getting merged as I see individual IO requests on
>> the wire of 2-4KB.
>>
>> Googling around this area seems to suggest this kernel behavior isn't
>> something I can disable. So if I honest to goodness *only* want 512B
>> IO, is ioengine=sg the best route?
>
> Try setting /sys/block/sdX/queue/nomerges to 2. That should completely
> disable merging.
Oh wait, you said you're using a RHEL 2.6.32 based kernel, right? That
isn't implemented there. You'll have to backport this patch:
commit 488991e28e55b4fbca8067edf0259f69d1a6f92c
Author: Alan D. Brunelle <Alan.Brunelle@xxxxxx>
Date: Fri Jan 29 09:04:08 2010 +0100
block: Added in stricter no merge semantics for block I/O
Cheers,
Jeff
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