dm stats: collect and report histogram of IO latencies
Add an option to dm statistics to collect and report a histogram of
IO latencies.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/Documentation/device-mapper/statistics.txt b/Documentation/device-mapper/statistics.txt
index ff6baea..4919b2d 100644
--- a/Documentation/device-mapper/statistics.txt
+++ b/Documentation/device-mapper/statistics.txt
@@ -13,9 +13,10 @@
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. 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
@@ -64,6 +65,18 @@
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