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Documentation / device-mapper / statistics.txt




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

1	DM statistics
2	=============
3	
4	Device Mapper supports the collection of I/O statistics on user-defined
5	regions of a DM device.	 If no regions are defined no statistics are
6	collected so there isn't any performance impact.  Only bio-based DM
7	devices are currently supported.
8	
9	Each user-defined region specifies a starting sector, length and step.
10	Individual statistics will be collected for each step-sized area within
11	the range specified.
12	
13	The I/O statistics counters for each step-sized area of a region are
14	in the same format as /sys/block/*/stat or /proc/diskstats (see:
15	Documentation/iostats.txt).  But two extra counters (12 and 13) are
16	provided: total time spent reading and writing.  When the histogram
17	argument is used, the 14th parameter is reported that represents the
18	histogram of latencies.  All these counters may be accessed by sending
19	the @stats_print message to the appropriate DM device via dmsetup.
20	
21	The reported times are in milliseconds and the granularity depends on
22	the kernel ticks.  When the option precise_timestamps is used, the
23	reported times are in nanoseconds.
24	
25	Each region has a corresponding unique identifier, which we call a
26	region_id, that is assigned when the region is created.	 The region_id
27	must be supplied when querying statistics about the region, deleting the
28	region, etc.  Unique region_ids enable multiple userspace programs to
29	request and process statistics for the same DM device without stepping
30	on each other's data.
31	
32	The creation of DM statistics will allocate memory via kmalloc or
33	fallback to using vmalloc space.  At most, 1/4 of the overall system
34	memory may be allocated by DM statistics.  The admin can see how much
35	memory is used by reading
36	/sys/module/dm_mod/parameters/stats_current_allocated_bytes
37	
38	Messages
39	========
40	
41	    @stats_create <range> <step>
42			[<number_of_optional_arguments> <optional_arguments>...]
43			[<program_id> [<aux_data>]]
44	
45		Create a new region and return the region_id.
46	
47		<range>
48		  "-" - whole device
49		  "<start_sector>+<length>" - a range of <length> 512-byte sectors
50					      starting with <start_sector>.
51	
52		<step>
53		  "<area_size>" - the range is subdivided into areas each containing
54				  <area_size> sectors.
55		  "/<number_of_areas>" - the range is subdivided into the specified
56					 number of areas.
57	
58		<number_of_optional_arguments>
59		  The number of optional arguments
60	
61		<optional_arguments>
62		  The following optional arguments are supported
63		  precise_timestamps - use precise timer with nanosecond resolution
64			instead of the "jiffies" variable.  When this argument is
65			used, the resulting times are in nanoseconds instead of
66			milliseconds.  Precise timestamps are a little bit slower
67			to obtain than jiffies-based timestamps.
68		  histogram:n1,n2,n3,n4,... - collect histogram of latencies.  The
69			numbers n1, n2, etc are times that represent the boundaries
70			of the histogram.  If precise_timestamps is not used, the
71			times are in milliseconds, otherwise they are in
72			nanoseconds.  For each range, the kernel will report the
73			number of requests that completed within this range. For
74			example, if we use "histogram:10,20,30", the kernel will
75			report four numbers a:b:c:d. a is the number of requests
76			that took 0-10 ms to complete, b is the number of requests
77			that took 10-20 ms to complete, c is the number of requests
78			that took 20-30 ms to complete and d is the number of
79			requests that took more than 30 ms to complete.
80	
81		<program_id>
82		  An optional parameter.  A name that uniquely identifies
83		  the userspace owner of the range.  This groups ranges together
84		  so that userspace programs can identify the ranges they
85		  created and ignore those created by others.
86		  The kernel returns this string back in the output of
87		  @stats_list message, but it doesn't use it for anything else.
88		  If we omit the number of optional arguments, program id must not
89		  be a number, otherwise it would be interpreted as the number of
90		  optional arguments.
91	
92		<aux_data>
93		  An optional parameter.  A word that provides auxiliary data
94		  that is useful to the client program that created the range.
95		  The kernel returns this string back in the output of
96		  @stats_list message, but it doesn't use this value for anything.
97	
98	    @stats_delete <region_id>
99	
100		Delete the region with the specified id.
101	
102		<region_id>
103		  region_id returned from @stats_create
104	
105	    @stats_clear <region_id>
106	
107		Clear all the counters except the in-flight i/o counters.
108	
109		<region_id>
110		  region_id returned from @stats_create
111	
112	    @stats_list [<program_id>]
113	
114		List all regions registered with @stats_create.
115	
116		<program_id>
117		  An optional parameter.
118		  If this parameter is specified, only matching regions
119		  are returned.
120		  If it is not specified, all regions are returned.
121	
122		Output format:
123		  <region_id>: <start_sector>+<length> <step> <program_id> <aux_data>
124		        precise_timestamps histogram:n1,n2,n3,...
125	
126		The strings "precise_timestamps" and "histogram" are printed only
127		if they were specified when creating the region.
128	
129	    @stats_print <region_id> [<starting_line> <number_of_lines>]
130	
131		Print counters for each step-sized area of a region.
132	
133		<region_id>
134		  region_id returned from @stats_create
135	
136		<starting_line>
137		  The index of the starting line in the output.
138		  If omitted, all lines are returned.
139	
140		<number_of_lines>
141		  The number of lines to include in the output.
142		  If omitted, all lines are returned.
143	
144		Output format for each step-sized area of a region:
145	
146		  <start_sector>+<length> counters
147	
148		  The first 11 counters have the same meaning as
149		  /sys/block/*/stat or /proc/diskstats.
150	
151		  Please refer to Documentation/iostats.txt for details.
152	
153		  1. the number of reads completed
154		  2. the number of reads merged
155		  3. the number of sectors read
156		  4. the number of milliseconds spent reading
157		  5. the number of writes completed
158		  6. the number of writes merged
159		  7. the number of sectors written
160		  8. the number of milliseconds spent writing
161		  9. the number of I/Os currently in progress
162		  10. the number of milliseconds spent doing I/Os
163		  11. the weighted number of milliseconds spent doing I/Os
164	
165		  Additional counters:
166		  12. the total time spent reading in milliseconds
167		  13. the total time spent writing in milliseconds
168	
169	    @stats_print_clear <region_id> [<starting_line> <number_of_lines>]
170	
171		Atomically print and then clear all the counters except the
172		in-flight i/o counters.	 Useful when the client consuming the
173		statistics does not want to lose any statistics (those updated
174		between printing and clearing).
175	
176		<region_id>
177		  region_id returned from @stats_create
178	
179		<starting_line>
180		  The index of the starting line in the output.
181		  If omitted, all lines are printed and then cleared.
182	
183		<number_of_lines>
184		  The number of lines to process.
185		  If omitted, all lines are printed and then cleared.
186	
187	    @stats_set_aux <region_id> <aux_data>
188	
189		Store auxiliary data aux_data for the specified region.
190	
191		<region_id>
192		  region_id returned from @stats_create
193	
194		<aux_data>
195		  The string that identifies data which is useful to the client
196		  program that created the range.  The kernel returns this
197		  string back in the output of @stats_list message, but it
198		  doesn't use this value for anything.
199	
200	Examples
201	========
202	
203	Subdivide the DM device 'vol' into 100 pieces and start collecting
204	statistics on them:
205	
206	  dmsetup message vol 0 @stats_create - /100
207	
208	Set the auxillary data string to "foo bar baz" (the escape for each
209	space must also be escaped, otherwise the shell will consume them):
210	
211	  dmsetup message vol 0 @stats_set_aux 0 foo\\ bar\\ baz
212	
213	List the statistics:
214	
215	  dmsetup message vol 0 @stats_list
216	
217	Print the statistics:
218	
219	  dmsetup message vol 0 @stats_print 0
220	
221	Delete the statistics:
222	
223	  dmsetup message vol 0 @stats_delete 0
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