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authorJens Axboe <jens.axboe@oracle.com>2009-02-02 07:02:31 -0500
committerJens Axboe <jens.axboe@oracle.com>2009-02-02 07:02:31 -0500
commitcbb5901b904e122139e97c6f4caed9b1f13c3455 (patch)
treeb8dc9f2f7108b54343d55169326f6d4eb333e498 /Documentation/block/queue-sysfs.txt
parentc52440a69df22dca69794936a91e2fb529a707fb (diff)
block: add text file detailing queue/ sysfs files
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
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1Queue sysfs files
2=================
3
4This text file will detail the queue files that are located in the sysfs tree
5for each block device. Note that stacked devices typically do not export
6any settings, since their queue merely functions are a remapping target.
7These files are the ones found in the /sys/block/xxx/queue/ directory.
8
9Files denoted with a RO postfix are readonly and the RW postfix means
10read-write.
11
12hw_sector_size (RO)
13-------------------
14This is the hardware sector size of the device, in bytes.
15
16max_hw_sectors_kb (RO)
17----------------------
18This is the maximum number of kilobytes supported in a single data transfer.
19
20max_sectors_kb (RW)
21-------------------
22This is the maximum number of kilobytes that the block layer will allow
23for a filesystem request. Must be smaller than or equal to the maximum
24size allowed by the hardware.
25
26nomerges (RW)
27-------------
28This enables the user to disable the lookup logic involved with IO merging
29requests in the block layer. Merging may still occur through a direct
301-hit cache, since that comes for (almost) free. The IO scheduler will not
31waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults
32to 0, enabling all merges.
33
34nr_requests (RW)
35----------------
36This controls how many requests may be allocated in the block layer for
37read or write requests. Note that the total allocated number may be twice
38this amount, since it applies only to reads or writes (not the accumulated
39sum).
40
41read_ahead_kb (RW)
42------------------
43Maximum number of kilobytes to read-ahead for filesystems on this block
44device.
45
46rq_affinity (RW)
47----------------
48If this option is enabled, the block layer will migrate request completions
49to the CPU that originally submitted the request. For some workloads
50this provides a significant reduction in CPU cycles due to caching effects.
51
52scheduler (RW)
53--------------
54When read, this file will display the current and available IO schedulers
55for this block device. The currently active IO scheduler will be enclosed
56in [] brackets. Writing an IO scheduler name to this file will switch
57control of this block device to that new IO scheduler. Note that writing
58an IO scheduler name to this file will attempt to load that IO scheduler
59module, if it isn't already present in the system.
60
61
62
63Jens Axboe <jens.axboe@oracle.com>, February 2009