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Documentation / block / queue-sysfs.txt




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Based on kernel version 4.8. Page generated on 2016-10-06 23:10 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 turn off the disk entropy contribution. Default
15	value of this file is '1'(on).
16	
17	dax (RO)
18	--------
19	This file indicates whether the device supports Direct Access (DAX),
20	used by CPU-addressable storage to bypass the pagecache.  It shows '1'
21	if true, '0' if not.
22	
23	discard_granularity (RO)
24	-----------------------
25	This shows the size of internal allocation of the device in bytes, if
26	reported by the device. A value of '0' means device does not support
27	the discard functionality.
28	
29	discard_max_hw_bytes (RO)
30	----------------------
31	Devices that support discard functionality may have internal limits on
32	the number of bytes that can be trimmed or unmapped in a single operation.
33	The discard_max_bytes parameter is set by the device driver to the maximum
34	number of bytes that can be discarded in a single operation. Discard
35	requests issued to the device must not exceed this limit. A discard_max_bytes
36	value of 0 means that the device does not support discard functionality.
37	
38	discard_max_bytes (RW)
39	----------------------
40	While discard_max_hw_bytes is the hardware limit for the device, this
41	setting is the software limit. Some devices exhibit large latencies when
42	large discards are issued, setting this value lower will make Linux issue
43	smaller discards and potentially help reduce latencies induced by large
44	discard operations.
45	
46	discard_zeroes_data (RO)
47	------------------------
48	When read, this file will show if the discarded block are zeroed by the
49	device or not. If its value is '1' the blocks are zeroed otherwise not.
50	
51	hw_sector_size (RO)
52	-------------------
53	This is the hardware sector size of the device, in bytes.
54	
55	io_poll (RW)
56	------------
57	When read, this file shows the total number of block IO polls and how
58	many returned success.  Writing '0' to this file will disable polling
59	for this device.  Writing any non-zero value will enable this feature.
60	
61	iostats (RW)
62	-------------
63	This file is used to control (on/off) the iostats accounting of the
64	disk.
65	
66	logical_block_size (RO)
67	-----------------------
68	This is the logical block size of the device, in bytes.
69	
70	max_hw_sectors_kb (RO)
71	----------------------
72	This is the maximum number of kilobytes supported in a single data transfer.
73	
74	max_integrity_segments (RO)
75	---------------------------
76	When read, this file shows the max limit of integrity segments as
77	set by block layer which a hardware controller can handle.
78	
79	max_sectors_kb (RW)
80	-------------------
81	This is the maximum number of kilobytes that the block layer will allow
82	for a filesystem request. Must be smaller than or equal to the maximum
83	size allowed by the hardware.
84	
85	max_segments (RO)
86	-----------------
87	Maximum number of segments of the device.
88	
89	max_segment_size (RO)
90	---------------------
91	Maximum segment size of the device.
92	
93	minimum_io_size (RO)
94	--------------------
95	This is the smallest preferred IO size reported by the device.
96	
97	nomerges (RW)
98	-------------
99	This enables the user to disable the lookup logic involved with IO
100	merging requests in the block layer. By default (0) all merges are
101	enabled. When set to 1 only simple one-hit merges will be tried. When
102	set to 2 no merge algorithms will be tried (including one-hit or more
103	complex tree/hash lookups).
104	
105	nr_requests (RW)
106	----------------
107	This controls how many requests may be allocated in the block layer for
108	read or write requests. Note that the total allocated number may be twice
109	this amount, since it applies only to reads or writes (not the accumulated
110	sum).
111	
112	To avoid priority inversion through request starvation, a request
113	queue maintains a separate request pool per each cgroup when
114	CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
115	per-block-cgroup request pool.  IOW, if there are N block cgroups,
116	each request queue may have up to N request pools, each independently
117	regulated by nr_requests.
118	
119	optimal_io_size (RO)
120	--------------------
121	This is the optimal IO size reported by the device.
122	
123	physical_block_size (RO)
124	------------------------
125	This is the physical block size of device, in bytes.
126	
127	read_ahead_kb (RW)
128	------------------
129	Maximum number of kilobytes to read-ahead for filesystems on this block
130	device.
131	
132	rotational (RW)
133	---------------
134	This file is used to stat if the device is of rotational type or
135	non-rotational type.
136	
137	rq_affinity (RW)
138	----------------
139	If this option is '1', the block layer will migrate request completions to the
140	cpu "group" that originally submitted the request. For some workloads this
141	provides a significant reduction in CPU cycles due to caching effects.
142	
143	For storage configurations that need to maximize distribution of completion
144	processing setting this option to '2' forces the completion to run on the
145	requesting cpu (bypassing the "group" aggregation logic).
146	
147	scheduler (RW)
148	--------------
149	When read, this file will display the current and available IO schedulers
150	for this block device. The currently active IO scheduler will be enclosed
151	in [] brackets. Writing an IO scheduler name to this file will switch
152	control of this block device to that new IO scheduler. Note that writing
153	an IO scheduler name to this file will attempt to load that IO scheduler
154	module, if it isn't already present in the system.
155	
156	write_cache (RW)
157	----------------
158	When read, this file will display whether the device has write back
159	caching enabled or not. It will return "write back" for the former
160	case, and "write through" for the latter. Writing to this file can
161	change the kernels view of the device, but it doesn't alter the
162	device state. This means that it might not be safe to toggle the
163	setting from "write back" to "write through", since that will also
164	eliminate cache flushes issued by the kernel.
165	
166	write_same_max_bytes (RO)
167	-------------------------
168	This is the number of bytes the device can write in a single write-same
169	command.  A value of '0' means write-same is not supported by this
170	device.
171	
172	
173	Jens Axboe <jens.axboe@oracle.com>, February 2009
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