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Documentation / block / queue-sysfs.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:44 EST.

1	Queue sysfs files
2	=================
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.
9	Files denoted with a RO postfix are readonly and the RW postfix means
10	read-write.
12	add_random (RW)
13	----------------
14	This file allows to turn off the disk entropy contribution. Default
15	value of this file is '1'(on).
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.
23	discard_max_hw_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.
32	discard_max_bytes (RW)
33	----------------------
34	While discard_max_hw_bytes is the hardware limit for the device, this
35	setting is the software limit. Some devices exhibit large latencies when
36	large discards are issued, setting this value lower will make Linux issue
37	smaller discards and potentially help reduce latencies induced by large
38	discard operations.
40	discard_zeroes_data (RO)
41	------------------------
42	When read, this file will show if the discarded block are zeroed by the
43	device or not. If its value is '1' the blocks are zeroed otherwise not.
45	hw_sector_size (RO)
46	-------------------
47	This is the hardware sector size of the device, in bytes.
49	iostats (RW)
50	-------------
51	This file is used to control (on/off) the iostats accounting of the
52	disk.
54	logical_block_size (RO)
55	-----------------------
56	This is the logcal block size of the device, in bytes.
58	max_hw_sectors_kb (RO)
59	----------------------
60	This is the maximum number of kilobytes supported in a single data transfer.
62	max_integrity_segments (RO)
63	---------------------------
64	When read, this file shows the max limit of integrity segments as
65	set by block layer which a hardware controller can handle.
67	max_sectors_kb (RW)
68	-------------------
69	This is the maximum number of kilobytes that the block layer will allow
70	for a filesystem request. Must be smaller than or equal to the maximum
71	size allowed by the hardware.
73	max_segments (RO)
74	-----------------
75	Maximum number of segments of the device.
77	max_segment_size (RO)
78	---------------------
79	Maximum segment size of the device.
81	minimum_io_size (RO)
82	--------------------
83	This is the smallest preferred IO size reported by the device.
85	nomerges (RW)
86	-------------
87	This enables the user to disable the lookup logic involved with IO
88	merging requests in the block layer. By default (0) all merges are
89	enabled. When set to 1 only simple one-hit merges will be tried. When
90	set to 2 no merge algorithms will be tried (including one-hit or more
91	complex tree/hash lookups).
93	nr_requests (RW)
94	----------------
95	This controls how many requests may be allocated in the block layer for
96	read or write requests. Note that the total allocated number may be twice
97	this amount, since it applies only to reads or writes (not the accumulated
98	sum).
100	To avoid priority inversion through request starvation, a request
101	queue maintains a separate request pool per each cgroup when
102	CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
103	per-block-cgroup request pool.  IOW, if there are N block cgroups,
104	each request queue may have up to N request pools, each independently
105	regulated by nr_requests.
107	optimal_io_size (RO)
108	--------------------
109	This is the optimal IO size reported by the device.
111	physical_block_size (RO)
112	------------------------
113	This is the physical block size of device, in bytes.
115	read_ahead_kb (RW)
116	------------------
117	Maximum number of kilobytes to read-ahead for filesystems on this block
118	device.
120	rotational (RW)
121	---------------
122	This file is used to stat if the device is of rotational type or
123	non-rotational type.
125	rq_affinity (RW)
126	----------------
127	If this option is '1', the block layer will migrate request completions to the
128	cpu "group" that originally submitted the request. For some workloads this
129	provides a significant reduction in CPU cycles due to caching effects.
131	For storage configurations that need to maximize distribution of completion
132	processing setting this option to '2' forces the completion to run on the
133	requesting cpu (bypassing the "group" aggregation logic).
135	scheduler (RW)
136	--------------
137	When read, this file will display the current and available IO schedulers
138	for this block device. The currently active IO scheduler will be enclosed
139	in [] brackets. Writing an IO scheduler name to this file will switch
140	control of this block device to that new IO scheduler. Note that writing
141	an IO scheduler name to this file will attempt to load that IO scheduler
142	module, if it isn't already present in the system.
146	Jens Axboe <jens.axboe@oracle.com>, February 2009
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