rsyslog statistic counter plugin omfile

Plugin – omfile (rsyslog 7.3.6+)

This plugin maintains statistics for each dynafile cache. Dynafile cache performance is critical for overall system performance, so reviewing these counters on a busy system (especially one experiencing performance problems) is advisable. The statistic is named “dynafile cache”, followed by the template name used for this dynafile action.

The following properties are maintained for each dynafile:

  • requests – total number of requests made to obtain a dynafile
  • level0 – requests for the current active file, so no real cache lookup needed to be done. These are extremely good.
  • missed – cache misses, where the required file did not reside in cache. Even with a perfect cache, there will be at least one miss per file. That happens when the file is being accessed for the first time and brought into cache. So “missed” will always be at least as large as the number of different files processed.
  • evicted – the number of times a file needed to be evicted from the cache as it ran out of space. These can simply happen when date-based files are used, and the previous date files are being removed from the cache as time progresses. It is better, though, to set an appropriate “closeTimeout” (counter described below), so that files are removed from the cache after they become no longer accessed. It is bad if active files need to be evicted from the cache. This is a very costly operation as an evict requires to close the file (thus a full flush, no matter of its buffer state) and a later access requires a re-open – and the eviction of another file, as the cache obviously has run out of free entries. If this happens frequently, it can severely affect performance. So a high eviction rate is a sign that the dynafile cache size should be increased. If it is already very high, it is recommended to re-think about the design of the file store, at least if the eviction process causes real performance problems.
  • maxused – the maximum number of cache entries ever used. This can be used to trim the cache down to a value that’s actually useful but does not waste resources. Note that when date-based files are used and rsyslog is run for an extended period of time, the cache gradually fills up to the max configured value as older files are migrated out of it. This will make “maxused” questionable after some time. Frequently enough purging the cache can prevent this (usually, once a day is sufficient).
  • closetimeouts –  available since 8.3.3 – tells how often a file was closed due to timeout settings (“closeTimeout” action parameter). These are cases where dynafiles or static files have been closed by rsyslog due to inactivity. Note that if no “closeTimeout” is specified for the action, this counter always is zero. A high or low number in itself doesn’t mean anything good or bad. It totally depends on the use case, so no general advise can be given.

Note that the dynafile caches are purged when a HUP is sent.

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