[PATCH] Document online io scheduler switching

We added the ability to change a block device's IO elevator scheduler both
at kernel boot and on-the-fly, but we only documented the elevator= boot
parameter.  Add a quick how-to on doing it on the fly.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jens Axboe <axboe@suse.de>

Documentation/block/switching-sched.txt

@@ -0,0 +1,22 @@
+As of the Linux 2.6.10 kernel, it is now possible to change the
+IO scheduler for a given block device on the fly (thus making it possible,
+for instance, to set the CFQ scheduler for the system default, but
+set a specific device to use the anticipatory or noop schedulers - which
+can improve that device's throughput).
+
+To set a specific scheduler, simply do this:
+
+echo SCHEDNAME > /sys/block/DEV/queue/scheduler
+
+where SCHEDNAME is the name of a defined IO scheduler, and DEV is the
+device name (hda, hdb, sga, or whatever you happen to have).
+
+The list of defined schedulers can be found by simply doing
+a "cat /sys/block/DEV/queue/scheduler" - the list of valid names
+will be displayed, with the currently selected scheduler in brackets:
+
+# cat /sys/block/hda/queue/scheduler
+noop anticipatory deadline [cfq]
+# echo anticipatory > /sys/block/hda/queue/scheduler
+# cat /sys/block/hda/queue/scheduler
+noop [anticipatory] deadline cfq