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2.3.4. Instances

Singular metrics have only one value and no associated instance domain. Some metrics contain a set of values that share a common set of semantics for a specific instance, such as one value per processor, or one value per disk spindle, and so on.


The PMDA implementation is solely responsible for choosing the instance identifiers that differentiate instances within the instance domain. The PMDA is also responsible for ensuring the uniqueness of instance identifiers in any instance domain, as described in Section, “Instance Identification”. Instance Identification

Consistent interpretation of instances and instance domains require a few simple rules to be followed by PMDA authors. The PMDA library provides a series of pmdaCache routines to assist.
  • Each internal instance identifier (numeric) must be a unique 31-bit number.
  • The external instance name (string) must be unique.
  • When the instance name contains a space, the name to the left of the first space (the short name) must also be unique.
  • Where an external instance name corresponds to some object or entity, there is an expectation that the association between the name and the object is fixed.
  • It is preferable, although not mandatory, for the association between and external instance name (string) and internal instance identifier (numeric) to be persistent.