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 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