Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics

[Jeff Moyer <jmoyer@redhat.com>]

Chris Mason <chris.mason@oracle.com> writes:

> On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote:
>> Andrea Arcangeli <aarcange@redhat.com> writes:
>> 
>> > On Mon, Jan 23, 2012 at 05:18:57PM +0100, Jan Kara wrote:
>> >> requst granularity. Sure, big requests will take longer to complete but
>> >> maximum request size is relatively low (512k by default) so writing maximum
>> >> sized request isn't that much slower than writing 4k. So it works OK in
>> >> practice.
>> >
>> > Totally unrelated to the writeback, but the merged big 512k requests
>> > actually adds up some measurable I/O scheduler latencies and they in
>> > turn slightly diminish the fairness that cfq could provide with
>> > smaller max request size. Probably even more measurable with SSDs (but
>> > then SSDs are even faster).
>> 
>> Are you speaking from experience?  If so, what workloads were negatively
>> affected by merging, and how did you measure that?
>
> https://lkml.org/lkml/2011/12/13/326
>
> This patch is another example, although for a slight different reason.
> I really have no idea yet what the right answer is in a generic sense,
> but you don't need a 512K request to see higher latencies from merging.

Well, this patch has almost nothing to with merging, right?  It's about
keeping I/O from the I/O scheduler for too long (or, prior to on-stack
plugging, it was about keeping the queue plugged for too long).  And,
I'm pretty sure that the testing involved there was with deadline or
noop, nothing to do with CFQ fairness.  ;-)

However, this does bring to light the bigger problem of optimizing for
the underlying storage and the workload requirements.  Some tuning can
be done in the I/O scheduler, but the plugging definitely circumvents
that a little bit.

-Jeff