On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote:

Andrea Arcangeli [off-list ref] writes:

quoted

On Mon, Jan 23, 2012 at 05:18:57PM +0100, Jan Kara wrote:

quoted

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.

-chris