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Documentation / block / queue-sysfs.txt




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Based on kernel version 3.2. Page generated on 2012-01-05 23:28 EST.

1	Queue sysfs files
2	=================
3	
4	This text file will detail the queue files that are located in the sysfs tree
5	for each block device. Note that stacked devices typically do not export
6	any settings, since their queue merely functions are a remapping target.
7	These files are the ones found in the /sys/block/xxx/queue/ directory.
8	
9	Files denoted with a RO postfix are readonly and the RW postfix means
10	read-write.
11	
12	hw_sector_size (RO)
13	-------------------
14	This is the hardware sector size of the device, in bytes.
15	
16	max_hw_sectors_kb (RO)
17	----------------------
18	This is the maximum number of kilobytes supported in a single data transfer.
19	
20	max_sectors_kb (RW)
21	-------------------
22	This is the maximum number of kilobytes that the block layer will allow
23	for a filesystem request. Must be smaller than or equal to the maximum
24	size allowed by the hardware.
25	
26	nomerges (RW)
27	-------------
28	This enables the user to disable the lookup logic involved with IO
29	merging requests in the block layer. By default (0) all merges are
30	enabled. When set to 1 only simple one-hit merges will be tried. When
31	set to 2 no merge algorithms will be tried (including one-hit or more
32	complex tree/hash lookups).
33	
34	nr_requests (RW)
35	----------------
36	This controls how many requests may be allocated in the block layer for
37	read or write requests. Note that the total allocated number may be twice
38	this amount, since it applies only to reads or writes (not the accumulated
39	sum).
40	
41	read_ahead_kb (RW)
42	------------------
43	Maximum number of kilobytes to read-ahead for filesystems on this block
44	device.
45	
46	rq_affinity (RW)
47	----------------
48	If this option is '1', the block layer will migrate request completions to the
49	cpu "group" that originally submitted the request. For some workloads this
50	provides a significant reduction in CPU cycles due to caching effects.
51	
52	For storage configurations that need to maximize distribution of completion
53	processing setting this option to '2' forces the completion to run on the
54	requesting cpu (bypassing the "group" aggregation logic).
55	
56	scheduler (RW)
57	--------------
58	When read, this file will display the current and available IO schedulers
59	for this block device. The currently active IO scheduler will be enclosed
60	in [] brackets. Writing an IO scheduler name to this file will switch
61	control of this block device to that new IO scheduler. Note that writing
62	an IO scheduler name to this file will attempt to load that IO scheduler
63	module, if it isn't already present in the system.
64	
65	
66	
67	Jens Axboe <jens.axboe@oracle.com>, February 2009
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