The performance-related file attributes PERFORMANCE, USAGE and DISK-WRITE were introduced in conjunction with the hiperfile concept:
PERFORMANCE Specifies the desired performance of I/O behavior for file access
USAGE Indicates whether the performance is requested for read access only, for write access only or for both types of access
DISK-WRITE Determines at which point during high-performance file processing data consistency for write operations is required.
PFA with SF pubsets
With SF pubsets, the performance-related file attributes control cache utilization: any file with higher performance requirements is automatically processed with caching. In this case, the USAGE and DISK-WRITE specifications and the cache type (VOLATILITY) assigned to the pubset determine whether read and/or write caching are performed.
If the file attributes PERFORMANCE=*HIGH, USAGE= *READ-WRITE and DISK-WRITE=*IMMEDIATE are selected for a file stored on an SF pubset with a non-secure cache (e.g. main memory cache), then only read caching is performed since the conditions for secure input/output write caching are not fulfilled. It is the user's task to ensure that the file attributes he/she specifies are compatible with the cache characteristics.
The output line CACHE-MEDIUM supplied by the command SHOW-MASTER-CATALOG-ENTRY INF=*USER indicates whether the pubset has been assigned a nonvolatile cache medium (CACHE-MEDIUM =NONVOLATILE). Only if this is the case do the file attributes USAGE=*WRITE and DISK-WRITE=*IMMEDIATE take effect.
For SF pubsets that have not been assigned a cache, the attributes PERFORMANCE, USAGE and DISK-WRITE are irrelevant. The performance behavior is exclusively determined by the location of the file.
These examples show that the user requires very detailed knowledge of the characteristics of the various media to be able to obtain the desired performance behavior.
PFA with SM pubsets
With SM pubsets, the performance-related file attributes are used to select the volume set that is best suitable as location of a particular file: When a new file is created, its performance attributes are used automatically to determine the volume set whose performance profile is best suited to meet the file requirements.
The performance profile of a volume set can be displayed by means of the SHOW-PUBSET-DEFINITION-FILE command. It is derived from the performance range of a volume set and the constraints on secure writing as a result of higher performance values. The complexity is a result of the fact that the performance profile has to characterize different cache types and disk types. The actual performance-relevant attributes of a volume set are based on the following considerations:
For instance, a volume set equipped with a cache offers standard performance (STD) for file processing without cache, and higher performance (HIGH or VERY-HIGH) for file processing with cache. This means that the performance range offered by a volume set equipped with a cache extends from STD to HIGH up to VERY-HIGH.
The SHOW-PUBSET-DEFINITION-FILE command supplies a list of performance values for volume sets. Note, however, that with nonsecure (volatile) caches, higher performance can only be offered for file processing that does not require data consistency after each write operation. This constraint with respect to higher performance is taken into account by the performance-relevant attribute WRITE-CONSISTENCY:
WRITE-CONSISTENCY=*BY-CLOSE indicates that higher performance can be offered only at the expense of secure writing.
Systems support has to ensure that the hardware or cache configurations that are appropriate for the performance profile of a volume set are made available (using the MODIFY-PUBSET-CACHE-ATTRIBUTES command).