[PATCH 3/3] Mention that the util-linux package provides an ionice command.

[Martin Steigerwald]

Am Montag, 28. November 2011 schrieb Martin Steigerwald:
> Hi jens und Vivek,
> 
> Vivek, I cc'd you, cause you wrote the new cfq-iosched.txt.
> 
> 
> In trying to understand how I/O priorities actually really work, I tried to
> dd with
> 
> rm nullen-id ; sync ; /usr/bin/time ionice -c3 dd if=/dev/zero of=nullen-id
> count=500 bs=1M conv=fsync
> 
> versus
> 
> rm nullen-rl; sync ; /usr/bin/time ionice -c1 -n0 dd if=/dev/zero
> of=nullen-rl count=500 bs=1M conv=fsync
> 
> concurrently. No differences. At first I was puzzled, then I thought maybe
> direct I/O makes a difference. So I tried with oflag=direct.
> 
> And it does.
> 
> Then I actually read the documentation block/ioprio.txt (3.1 here):
> > With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
> > priorities are supported for reads on files.  This enables users to io
> > nice processes or process groups, similar to what has been possible with
> > cpu scheduling for ages.  This document mainly details the current
> > possibilities with cfq; other io schedulers do not support io priorities
> > thus far.
> 
> According to it I/O priorities will even only work on reads. Is that
> correct? I mean they do work on reads, I tested it, but *only* on reads?
> 
> From what I see here, it also works for direct I/O write requests
> 
> So from what I conclude is that CFQ I/O priorities work for all requests
> that are issued via synchronous system calls, but not for those issued via
> asynchronous calls, i. e. everything that goes through the pagecache.
> 
> Is that correct?
> 
> 
> Vivek, one thing on cfq-iosched.txt: Could slice_idle=0 make sense on SSDs?
> Later on you write that there are some SSD optimizations in place that cut
> down idling already.

And mentioned that ionice is available from the util-linux package: