[dm-devel] dm-mpath request merging concerns [was: Re: It's time to put together the schedule]

[Jeff Moyer]

Mike Snitzer <snitzer at redhat.com> writes:

> Jens and/or Jeff Moyer, are there any knobs that you'd suggest to try to
> promote request merging on a really fast block device?  Any scheduler
> and knobs you'd suggest would be appreciated.

There's a small chance that CFQ does what you want.  It has logic to
detect when multiple processes are submitting interleaved I/O, and it
will try to wait until requests are merged before dispatching (see
the comment in cfq_rq_enqueued).

Aside from that, make sure /sys/block/<dev>/queue/rotational is set to 1.

Cheers,
Jeff