Understanding the request histogram facility

Use the mmpmon rhist command requests to control the request histogram facility.

The request histogram facility tallies I/O operations using a set of counters. Counters for reads and writes are kept separately. They are categorized according to a pattern that might be customized by the user. A default pattern is also provided. The size range and latency range input parameters to the rhist nr request are used to define the pattern.

The first time that you run the rhist requests, assess if there is a noticeable performance degradation. Collecting histogram data might cause performance degradation. This action is possible once the histogram facility is enabled, but is probably not noticed while the commands themselves are running. It is more of a long-term issue as the GPFS daemon runs with histograms enabled.

The histogram lock is used to prevent two rhist requests from being processed simultaneously. If an rhist request fails with an _rc_ of 16, then the lock is in use. Reissue the request.

The histogram data survives file system mounts and unmounts. To reset this data, use the rhist reset request.

Table 1 describes the rhist requests: