On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote: > Andrea Arcangeli <aarcange@xxxxxxxxxx> 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. -chris -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
