From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeff Moyer <jmoyer@redhat.com>
Subject: Re: dm-mpath request merging concerns [was: Re: It's
	time to put together the schedule]
Date: Tue, 24 Feb 2015 09:35:35 -0500
Message-ID: <x49k2z7l6mg.fsf@segfault.boston.devel.redhat.com>
References: <54EAD453.6040907@suse.de> <20150223135057.GA3362@redhat.com>
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	<20150223224637.GB5503@redhat.com>
	<20150223221438.GX11463@ask-08.lab.msp.redhat.com>
	<20150224003900.GA6421@redhat.com>
	<20150224003816.GY11463@ask-08.lab.msp.redhat.com>
	<20150224020231.GA6761@redhat.com>
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Mike Snitzer <snitzer@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