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

[Jeff Moyer <jmoyer redhat com>]

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?

Cheers,
Jeff