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Add support for creating, parsing, and reporting dm-stats latency
histograms on kernels that support precise_timestamps.
Histograms are specified as a series of time values that give the
boundaries of the bins into which I/O counts accumulate (with
implicit lower and upper bounds on the first and last bins).
A new type, struct dm_histogram, is introduced to represent
histogram values and bin boundaries.
The boundary values may be given as either a string of values (with
optional unit suffixes) or as a zero terminated array of uint64_t
values expressing boundary times in nanoseconds.
A new bounds argument is added to dm_stats_create_region() which
accepts a pointer to a struct dm_histogram initialised with bounds
values.
Histogram data associated with a region is parsed during a call to
dm_stats_populate() and used to build a table of histogram values
that are pointed to from the containing area's counter set. The
histogram for a specified area may then be obtained and interogated
for values and properties.
This relies on kernel support to provide the boundary values in
a @stats_list response: this will be present in 4.3 and 4.2-stable. A
check for a minimum driver version of 4.33.0 is implemented to ensure
that this is present (4.32.0 has the necessary precise_timestamps and
histogram features but is unable to report these via @stats_list).
Access methods are provided to retrieve histogram values and bounds
as well as simple string representations of the counts and bin
boundaries. Methods are also available to return the total count
for a histogram and the relative value (as a dm_percent_t) of a
specified bin.
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