Processing of rhist nr

The rhist nr request changes the request histogram facility request size and latency ranges.

Processing of rhist nr is as follows:

  1. The size range and latency range operands are parsed and checked for validity. If they are not valid, an error is returned and processing terminates.
  2. The histogram facility is disabled.
  3. The new ranges are created, by defining the following histogram counters:
    1. Two sets, one for read and one for write.
    2. Within each set, one category for each size range.
    3. Within each size range category, one counter for each latency range.

      For example, if the user specifies 11 numbers for the size range operand and 2 numbers for the latency range operand, this produces 12 size ranges, each having 3 latency ranges, because there is one additional range for the top endpoint. The total number of counters is 72: 36 read counters and 36 write counters.

  4. The new ranges are made current.
  5. The old ranges are discarded. Any accumulated histogram data is lost.
The histogram facility must be explicitly enabled again using rhist on to begin collecting histogram data using the new ranges.

The mmpmon command does not have the ability to collect data only for read operations, or only for write operations. The mmpmon command does not have the ability to specify size or latency ranges that have different values for read and write operations. The mmpmon command does not have the ability to specify latency ranges that are unique to a given size range.

For more information, see Specifying the size ranges for I/O histograms and Specifying the latency ranges for I/O.

Parent topic: Changing the request histogram facility request size and latency ranges