diff options
Diffstat (limited to 'Documentation/device-mapper/statistics.txt')
| -rw-r--r-- | Documentation/device-mapper/statistics.txt | 41 |
1 files changed, 37 insertions, 4 deletions
diff --git a/Documentation/device-mapper/statistics.txt b/Documentation/device-mapper/statistics.txt index 2a1673adc200..4919b2dfd1b3 100644 --- a/Documentation/device-mapper/statistics.txt +++ b/Documentation/device-mapper/statistics.txt @@ -13,9 +13,14 @@ the range specified. The I/O statistics counters for each step-sized area of a region are in the same format as /sys/block/*/stat or /proc/diskstats (see: Documentation/iostats.txt). But two extra counters (12 and 13) are -provided: total time spent reading and writing in milliseconds. All -these counters may be accessed by sending the @stats_print message to -the appropriate DM device via dmsetup. +provided: total time spent reading and writing. When the histogram +argument is used, the 14th parameter is reported that represents the +histogram of latencies. All these counters may be accessed by sending +the @stats_print message to the appropriate DM device via dmsetup. + +The reported times are in milliseconds and the granularity depends on +the kernel ticks. When the option precise_timestamps is used, the +reported times are in nanoseconds. Each region has a corresponding unique identifier, which we call a region_id, that is assigned when the region is created. The region_id @@ -33,7 +38,9 @@ memory is used by reading Messages ======== - @stats_create <range> <step> [<program_id> [<aux_data>]] + @stats_create <range> <step> + [<number_of_optional_arguments> <optional_arguments>...] + [<program_id> [<aux_data>]] Create a new region and return the region_id. @@ -48,6 +55,29 @@ Messages "/<number_of_areas>" - the range is subdivided into the specified number of areas. + <number_of_optional_arguments> + The number of optional arguments + + <optional_arguments> + The following optional arguments are supported + precise_timestamps - use precise timer with nanosecond resolution + instead of the "jiffies" variable. When this argument is + used, the resulting times are in nanoseconds instead of + milliseconds. Precise timestamps are a little bit slower + to obtain than jiffies-based timestamps. + histogram:n1,n2,n3,n4,... - collect histogram of latencies. The + numbers n1, n2, etc are times that represent the boundaries + of the histogram. If precise_timestamps is not used, the + times are in milliseconds, otherwise they are in + nanoseconds. For each range, the kernel will report the + number of requests that completed within this range. For + example, if we use "histogram:10,20,30", the kernel will + report four numbers a:b:c:d. a is the number of requests + that took 0-10 ms to complete, b is the number of requests + that took 10-20 ms to complete, c is the number of requests + that took 20-30 ms to complete and d is the number of + requests that took more than 30 ms to complete. + <program_id> An optional parameter. A name that uniquely identifies the userspace owner of the range. This groups ranges together @@ -55,6 +85,9 @@ Messages created and ignore those created by others. The kernel returns this string back in the output of @stats_list message, but it doesn't use it for anything else. + If we omit the number of optional arguments, program id must not + be a number, otherwise it would be interpreted as the number of + optional arguments. <aux_data> An optional parameter. A word that provides auxiliary data |