This patch modularizes the io schedulers completely, allowing them to be
modular.  Additionally it enables online switching of io schedulers.  See
also http://lwn.net/Articles/102593/ .


There's a scheduler file in the sysfs directory for the block device
queue:

axboe@router:/sys/block/hda/queue> ls
iosched            max_sectors_kb  read_ahead_kb
max_hw_sectors_kb  nr_requests     scheduler

If you list the contents of the file, it will show available schedulers
and the active one:

axboe@router:/sys/block/hda/queue> cat scheduler
[cfq]

Lets load a few more.

router:/sys/block/hda/queue # modprobe deadline-iosched
router:/sys/block/hda/queue # modprobe as-iosched
router:/sys/block/hda/queue # cat scheduler
[cfq] deadline anticipatory

Changing is done with

router:/sys/block/hda/queue # echo deadline > scheduler
router:/sys/block/hda/queue # cat scheduler
cfq [deadline] anticipatory

deadline is now the new active io scheduler for hda.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>