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Documentation: update tunable options in block/cfq-iosched.txt

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Update tunable options in block/cfq-iosched.txt.

Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Namjae Jeon 2012-08-09 15:27:29 +02:00 committed by Jens Axboe
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commit 2792d87193
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Documentation/block/cfq-iosched.txt
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@ -1,3 +1,14 @@
CFQ (Complete Fairness Queueing)
===============================
The main aim of CFQ scheduler is to provide a fair allocation of the disk
I/O bandwidth for all the processes which requests an I/O operation.
CFQ maintains the per process queue for the processes which request I/O
operation(syncronous requests). In case of asynchronous requests, all the
requests from all the processes are batched together according to their
process's I/O priority.
CFQ ioscheduler tunables
========================
@ -25,6 +36,72 @@ there are multiple spindles behind single LUN (Host based hardware RAID
controller or for storage arrays), setting slice_idle=0 might end up in better
throughput and acceptable latencies.
back_seek_max
-------------
This specifies, given in Kbytes, the maximum "distance" for backward seeking.
The distance is the amount of space from the current head location to the
sectors that are backward in terms of distance.
This parameter allows the scheduler to anticipate requests in the "backward"
direction and consider them as being the "next" if they are within this
distance from the current head location.
back_seek_penalty
-----------------
This parameter is used to compute the cost of backward seeking. If the
backward distance of request is just 1/back_seek_penalty from a "front"
request, then the seeking cost of two requests is considered equivalent.
So scheduler will not bias toward one or the other request (otherwise scheduler
will bias toward front request). Default value of back_seek_penalty is 2.
fifo_expire_async
-----------------
This parameter is used to set the timeout of asynchronous requests. Default
value of this is 248ms.
fifo_expire_sync
----------------
This parameter is used to set the timeout of synchronous requests. Default
value of this is 124ms. In case to favor synchronous requests over asynchronous
one, this value should be decreased relative to fifo_expire_async.
slice_async
-----------
This parameter is same as of slice_sync but for asynchronous queue. The
default value is 40ms.
slice_async_rq
--------------
This parameter is used to limit the dispatching of asynchronous request to
device request queue in queue's slice time. The maximum number of request that
are allowed to be dispatched also depends upon the io priority. Default value
for this is 2.
slice_sync
----------
When a queue is selected for execution, the queues IO requests are only
executed for a certain amount of time(time_slice) before switching to another
queue. This parameter is used to calculate the time slice of synchronous
queue.
time_slice is computed using the below equation:-
time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
time_slice of synchronous queue, increase the value of slice_sync. Default
value is 100ms.
quantum
-------
This specifies the number of request dispatched to the device queue. In a
queue's time slice, a request will not be dispatched if the number of request
in the device exceeds this parameter. This parameter is used for synchronous
request.
In case of storage with several disk, this setting can limit the parallel
processing of request. Therefore, increasing the value can imporve the
performace although this can cause the latency of some I/O to increase due
to more number of requests.
CFQ IOPS Mode for group scheduling
===================================
Basic CFQ design is to provide priority based time slices. Higher priority