statistics.rst (7753B)
1============= 2DM statistics 3============= 4 5Device Mapper supports the collection of I/O statistics on user-defined 6regions of a DM device. If no regions are defined no statistics are 7collected so there isn't any performance impact. Only bio-based DM 8devices are currently supported. 9 10Each user-defined region specifies a starting sector, length and step. 11Individual statistics will be collected for each step-sized area within 12the range specified. 13 14The I/O statistics counters for each step-sized area of a region are 15in the same format as `/sys/block/*/stat` or `/proc/diskstats` (see: 16Documentation/admin-guide/iostats.rst). But two extra counters (12 and 13) are 17provided: total time spent reading and writing. When the histogram 18argument is used, the 14th parameter is reported that represents the 19histogram of latencies. All these counters may be accessed by sending 20the @stats_print message to the appropriate DM device via dmsetup. 21 22The reported times are in milliseconds and the granularity depends on 23the kernel ticks. When the option precise_timestamps is used, the 24reported times are in nanoseconds. 25 26Each region has a corresponding unique identifier, which we call a 27region_id, that is assigned when the region is created. The region_id 28must be supplied when querying statistics about the region, deleting the 29region, etc. Unique region_ids enable multiple userspace programs to 30request and process statistics for the same DM device without stepping 31on each other's data. 32 33The creation of DM statistics will allocate memory via kmalloc or 34fallback to using vmalloc space. At most, 1/4 of the overall system 35memory may be allocated by DM statistics. The admin can see how much 36memory is used by reading: 37 38 /sys/module/dm_mod/parameters/stats_current_allocated_bytes 39 40Messages 41======== 42 43 @stats_create <range> <step> [<number_of_optional_arguments> <optional_arguments>...] [<program_id> [<aux_data>]] 44 Create a new region and return the region_id. 45 46 <range> 47 "-" 48 whole device 49 "<start_sector>+<length>" 50 a range of <length> 512-byte sectors 51 starting with <start_sector>. 52 53 <step> 54 "<area_size>" 55 the range is subdivided into areas each containing 56 <area_size> sectors. 57 "/<number_of_areas>" 58 the range is subdivided into the specified 59 number of areas. 60 61 <number_of_optional_arguments> 62 The number of optional arguments 63 64 <optional_arguments> 65 The following optional arguments are supported: 66 67 precise_timestamps 68 use precise timer with nanosecond resolution 69 instead of the "jiffies" variable. When this argument is 70 used, the resulting times are in nanoseconds instead of 71 milliseconds. Precise timestamps are a little bit slower 72 to obtain than jiffies-based timestamps. 73 histogram:n1,n2,n3,n4,... 74 collect histogram of latencies. The 75 numbers n1, n2, etc are times that represent the boundaries 76 of the histogram. If precise_timestamps is not used, the 77 times are in milliseconds, otherwise they are in 78 nanoseconds. For each range, the kernel will report the 79 number of requests that completed within this range. For 80 example, if we use "histogram:10,20,30", the kernel will 81 report four numbers a:b:c:d. a is the number of requests 82 that took 0-10 ms to complete, b is the number of requests 83 that took 10-20 ms to complete, c is the number of requests 84 that took 20-30 ms to complete and d is the number of 85 requests that took more than 30 ms to complete. 86 87 <program_id> 88 An optional parameter. A name that uniquely identifies 89 the userspace owner of the range. This groups ranges together 90 so that userspace programs can identify the ranges they 91 created and ignore those created by others. 92 The kernel returns this string back in the output of 93 @stats_list message, but it doesn't use it for anything else. 94 If we omit the number of optional arguments, program id must not 95 be a number, otherwise it would be interpreted as the number of 96 optional arguments. 97 98 <aux_data> 99 An optional parameter. A word that provides auxiliary data 100 that is useful to the client program that created the range. 101 The kernel returns this string back in the output of 102 @stats_list message, but it doesn't use this value for anything. 103 104 @stats_delete <region_id> 105 Delete the region with the specified id. 106 107 <region_id> 108 region_id returned from @stats_create 109 110 @stats_clear <region_id> 111 Clear all the counters except the in-flight i/o counters. 112 113 <region_id> 114 region_id returned from @stats_create 115 116 @stats_list [<program_id>] 117 List all regions registered with @stats_create. 118 119 <program_id> 120 An optional parameter. 121 If this parameter is specified, only matching regions 122 are returned. 123 If it is not specified, all regions are returned. 124 125 Output format: 126 <region_id>: <start_sector>+<length> <step> <program_id> <aux_data> 127 precise_timestamps histogram:n1,n2,n3,... 128 129 The strings "precise_timestamps" and "histogram" are printed only 130 if they were specified when creating the region. 131 132 @stats_print <region_id> [<starting_line> <number_of_lines>] 133 Print counters for each step-sized area of a region. 134 135 <region_id> 136 region_id returned from @stats_create 137 138 <starting_line> 139 The index of the starting line in the output. 140 If omitted, all lines are returned. 141 142 <number_of_lines> 143 The number of lines to include in the output. 144 If omitted, all lines are returned. 145 146 Output format for each step-sized area of a region: 147 148 <start_sector>+<length> 149 counters 150 151 The first 11 counters have the same meaning as 152 `/sys/block/*/stat or /proc/diskstats`. 153 154 Please refer to Documentation/admin-guide/iostats.rst for details. 155 156 1. the number of reads completed 157 2. the number of reads merged 158 3. the number of sectors read 159 4. the number of milliseconds spent reading 160 5. the number of writes completed 161 6. the number of writes merged 162 7. the number of sectors written 163 8. the number of milliseconds spent writing 164 9. the number of I/Os currently in progress 165 10. the number of milliseconds spent doing I/Os 166 11. the weighted number of milliseconds spent doing I/Os 167 168 Additional counters: 169 170 12. the total time spent reading in milliseconds 171 13. the total time spent writing in milliseconds 172 173 @stats_print_clear <region_id> [<starting_line> <number_of_lines>] 174 Atomically print and then clear all the counters except the 175 in-flight i/o counters. Useful when the client consuming the 176 statistics does not want to lose any statistics (those updated 177 between printing and clearing). 178 179 <region_id> 180 region_id returned from @stats_create 181 182 <starting_line> 183 The index of the starting line in the output. 184 If omitted, all lines are printed and then cleared. 185 186 <number_of_lines> 187 The number of lines to process. 188 If omitted, all lines are printed and then cleared. 189 190 @stats_set_aux <region_id> <aux_data> 191 Store auxiliary data aux_data for the specified region. 192 193 <region_id> 194 region_id returned from @stats_create 195 196 <aux_data> 197 The string that identifies data which is useful to the client 198 program that created the range. The kernel returns this 199 string back in the output of @stats_list message, but it 200 doesn't use this value for anything. 201 202Examples 203======== 204 205Subdivide the DM device 'vol' into 100 pieces and start collecting 206statistics on them:: 207 208 dmsetup message vol 0 @stats_create - /100 209 210Set the auxiliary data string to "foo bar baz" (the escape for each 211space must also be escaped, otherwise the shell will consume them):: 212 213 dmsetup message vol 0 @stats_set_aux 0 foo\\ bar\\ baz 214 215List the statistics:: 216 217 dmsetup message vol 0 @stats_list 218 219Print the statistics:: 220 221 dmsetup message vol 0 @stats_print 0 222 223Delete the statistics:: 224 225 dmsetup message vol 0 @stats_delete 0