Andrea Arcangeli <aarcange@xxxxxxxxxx> writes: > On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote: >> Are you speaking from experience? If so, what workloads were negatively >> affected by merging, and how did you measure that? > > Any workload where two processes compete for accessing the same disk > and one process writes big requests (usually async writes), the other > small (usually sync reads). The one with the small 4k requests > (usually reads) gets some artificial latency if the big requests are > 512k. Vivek did a recent measurement to verify the issue is still > there, and it's basically an hardware issue. Software can't do much > other than possibly reducing the max request size when we notice such > an I/O pattern coming in cfq. I did old measurements that's how I knew > it, but they were so ancient they're worthless by now, this is why > Vivek had to repeat it to verify before we could assume it still > existed on recent hardware. > > These days with cgroups it may be a bit more relevant as max write > bandwidth may be secondary to latency/QoS. Thanks, Vivek was able to point me at the old thread: http://www.spinics.net/lists/linux-fsdevel/msg44191.html Cheers, Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
