[PATCH] deadline I/O scheduler documentation

Document the deadline scheduler and its tunables.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/block/deadline-iosched.txt

+Deadline IO scheduler tunables
+==============================
+
+This little file attempts to document how the deadline io scheduler works.
+In particular, it will clarify the meaning of the exposed tunables that may be
+of interest to power users.
+
+Each io queue has a set of io scheduler tunables associated with it. These
+tunables control how the io scheduler works. You can find these entries
+in:
+
+/sys/block/<device>/iosched
+
+assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
+you can do so by typing:
+
+# mount none /sys -t sysfs
+
+
+********************************************************************************
+
+
+read_expire	(in ms)
+-----------
+
+The goal of the deadline io scheduler is to attempt to guarentee a start
+service time for a request. As we focus mainly on read latencies, this is
+tunable. When a read request first enters the io scheduler, it is assigned
+a deadline that is the current time + the read_expire value in units of
+miliseconds.
+
+
+write_expire	(in ms)
+-----------
+
+Similar to read_expire mentioned above, but for writes.
+
+
+fifo_batch
+----------
+
+When a read request expires its deadline, we must move some requests from
+the sorted io scheduler list to the block device dispatch queue. fifo_batch
+controls how many requests we move, based on the cost of each request. A
+request is either qualified as a seek or a stream. The io scheduler knows
+the last request that was serviced by the drive (or will be serviced right
+before this one). See seek_cost and stream_unit.
+
+
+write_starved	(number of dispatches)
+-------------
+
+When we have to move requests from the io scheduler queue to the block
+device dispatch queue, we always give a preference to reads. However, we
+don't want to starve writes indefinitely either. So writes_starved controls
+how many times we give preference to reads over writes. When that has been
+done writes_starved number of times, we dispatch some writes based on the
+same criteria as reads.
+
+
+front_merges	(bool)
+------------
+
+Sometimes it happens that a request enters the io scheduler that is contigious
+with a request that is already on the queue. Either it fits in the back of that
+request, or it fits at the front. That is called either a back merge candidate
+or a front merge candidate. Due to the way files are typically laid out,
+back merges are much more common than front merges. For some work loads, you
+may even know that it is a waste of time to spend any time attempting to
+front merge requests. Setting front_merges to 0 disables this functionality.
+Front merges may still occur due to the cached last_merge hint, but since
+that comes at basically 0 cost we leave that on. We simply disable the
+rbtree front sector lookup when the io scheduler merge function is called.
+
+
+Nov 11 2002, Jens Axboe <axboe@suse.de>
+
+