Policy rules

A policy rule is an SQL-like statement that tells GPFS what to do with the data for a file in a specific storage pool if the file meets specific criteria. A rule can apply to any file being created or only to files being created within a specific fileset or group of filesets.

A policy rule specifies one or more conditions that, when true, cause the rule to be applied. Conditions can be specified by SQL expressions, which can include SQL functions, variables, and file attributes. Some of the many available file attributes are shown in the following list. For more information, see File attributes in SQL expressions:

Date and time when the rule is evaluated, that is, the current date and time
Date and time when the file was last accessed
Date and time when the file was last modified
Fileset name
File name or extension
File size
User ID and group ID

Note: Some file attributes are not valid in all types of policy rules.

GPFS evaluates policy rules in order, from first to last, as they appear in the policy. The first rule that matches determines what is to be done with that file. For example, when a client creates a file, GPFS scans the list of rules in the active file placement policy to determine which rule applies to the file. When a rule applies to the file, GPFS stops processing the rules and assigns the file to the appropriate storage pool. If no rule applies, an EINVAL error code is returned to the application.

There are nine types of policy rules that allow you to define specific actions that GPFS will implement on the file data. Each rule has clauses that control candidate selection, namely when the rule is allowed to match a file, what files it will match, the order to operate on the matching files and additional attributes to show for each candidate file. Different clauses are permitted on different rules based upon the semantics of the rule.