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Documentation / block / queue-sysfs.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 22:55 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	add_random (RW)
13	----------------
14	This file allows to trun off the disk entropy contribution. Default
15	value of this file is '1'(on).
16	
17	discard_granularity (RO)
18	-----------------------
19	This shows the size of internal allocation of the device in bytes, if
20	reported by the device. A value of '0' means device does not support
21	the discard functionality.
22	
23	discard_max_bytes (RO)
24	----------------------
25	Devices that support discard functionality may have internal limits on
26	the number of bytes that can be trimmed or unmapped in a single operation.
27	The discard_max_bytes parameter is set by the device driver to the maximum
28	number of bytes that can be discarded in a single operation. Discard
29	requests issued to the device must not exceed this limit. A discard_max_bytes
30	value of 0 means that the device does not support discard functionality.
31	
32	discard_zeroes_data (RO)
33	------------------------
34	When read, this file will show if the discarded block are zeroed by the
35	device or not. If its value is '1' the blocks are zeroed otherwise not.
36	
37	hw_sector_size (RO)
38	-------------------
39	This is the hardware sector size of the device, in bytes.
40	
41	iostats (RW)
42	-------------
43	This file is used to control (on/off) the iostats accounting of the
44	disk.
45	
46	logical_block_size (RO)
47	-----------------------
48	This is the logcal block size of the device, in bytes.
49	
50	max_hw_sectors_kb (RO)
51	----------------------
52	This is the maximum number of kilobytes supported in a single data transfer.
53	
54	max_integrity_segments (RO)
55	---------------------------
56	When read, this file shows the max limit of integrity segments as
57	set by block layer which a hardware controller can handle.
58	
59	max_sectors_kb (RW)
60	-------------------
61	This is the maximum number of kilobytes that the block layer will allow
62	for a filesystem request. Must be smaller than or equal to the maximum
63	size allowed by the hardware.
64	
65	max_segments (RO)
66	-----------------
67	Maximum number of segments of the device.
68	
69	max_segment_size (RO)
70	---------------------
71	Maximum segment size of the device.
72	
73	minimum_io_size (RO)
74	--------------------
75	This is the smallest preferred io size reported by the device.
76	
77	nomerges (RW)
78	-------------
79	This enables the user to disable the lookup logic involved with IO
80	merging requests in the block layer. By default (0) all merges are
81	enabled. When set to 1 only simple one-hit merges will be tried. When
82	set to 2 no merge algorithms will be tried (including one-hit or more
83	complex tree/hash lookups).
84	
85	nr_requests (RW)
86	----------------
87	This controls how many requests may be allocated in the block layer for
88	read or write requests. Note that the total allocated number may be twice
89	this amount, since it applies only to reads or writes (not the accumulated
90	sum).
91	
92	To avoid priority inversion through request starvation, a request
93	queue maintains a separate request pool per each cgroup when
94	CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
95	per-block-cgroup request pool.  IOW, if there are N block cgroups,
96	each request queue may have upto N request pools, each independently
97	regulated by nr_requests.
98	
99	optimal_io_size (RO)
100	--------------------
101	This is the optimal io size reported by the device.
102	
103	physical_block_size (RO)
104	------------------------
105	This is the physical block size of device, in bytes.
106	
107	read_ahead_kb (RW)
108	------------------
109	Maximum number of kilobytes to read-ahead for filesystems on this block
110	device.
111	
112	rotational (RW)
113	---------------
114	This file is used to stat if the device is of rotational type or
115	non-rotational type.
116	
117	rq_affinity (RW)
118	----------------
119	If this option is '1', the block layer will migrate request completions to the
120	cpu "group" that originally submitted the request. For some workloads this
121	provides a significant reduction in CPU cycles due to caching effects.
122	
123	For storage configurations that need to maximize distribution of completion
124	processing setting this option to '2' forces the completion to run on the
125	requesting cpu (bypassing the "group" aggregation logic).
126	
127	scheduler (RW)
128	--------------
129	When read, this file will display the current and available IO schedulers
130	for this block device. The currently active IO scheduler will be enclosed
131	in [] brackets. Writing an IO scheduler name to this file will switch
132	control of this block device to that new IO scheduler. Note that writing
133	an IO scheduler name to this file will attempt to load that IO scheduler
134	module, if it isn't already present in the system.
135	
136	
137	
138	Jens Axboe <jens.axboe@oracle.com>, February 2009
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