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

[Andrea Arcangeli <aarcange@redhat.com>]

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.