Systems support defines the performance profile of a volume set with the PERFORMANCE-ATTR operand in the MODIFY-PUBSET-DEFINITION-FILE command. Systems support is responsible for defining the volume set performance profile in such a way that it correctly defines the real circumstances. PERFORMANCE-ATTR defines a performance spectrum (PERFORMANCE=) and the specification of restrictions with respect to the write consistency (WRITE-CONSISTENCY=).
Performance spectrum:
A volume set with a cache can generally cover a spectrum of different user performance requirements (STD, HIGH, VERY-HIGH). It is therefore suitable for use with files requiring PERFORMANCE=*STD, *HIGH or *VERY-HIGH. The different performance values correspond to different types of cache usage which are possible when processing files:PERFORMANCE=*STD (no cache usage)
PERFORMANCE=*HIGH (caching with migration)
PERFORMANCE=*VERY-HIGH (caching without migration)
The type of cache usage can be changed to suit the individual performance requirements of the separate files.
It is not possible to simultaneously assign several different caches to one volume set.Write consistency:
The cache media differ in their write consistency (WRITE-CONSISTENCY=*BY-CLOSE/*IMMEDIATE).
WRITE-CONSISTENCY=*IMMEDIATE means that enhanced performance can be used on the volume set without restrictions. WRITE-CONSISTENCY= *BY-CLOSE means that enhanced performance is only used for write operations if data consistency at CLOSE time is sufficient and is requested as such.
A cache is considered to be secure if the risk of failure for output data buffered in the cache is not higher than if it were written immediately to disk.
Volatile caches can generally be used for performance enhancement of read file accesses. They should only be used for writes if the user accepts the reduced write consistency and expresses this by setting the file attribute DISK-WRITE=*BY-CLOSE.
Recommendations for the performance profile
The settings recommended in the following table for the different cache media can therefore only be taken as an aid to orientation for the performance spectrum since they do not take the conditions of real configurations into account. It is, however, possible to make recommendations for write consistency with enhanced performance which can be used in any configuration.
Volume set configuration | Recommended performance (Performance spectrum) | Recommended performance with enhanced performance |
Volume set made up of volumes with no special performance properties and with no cache assigned | STD | irrelevant |
Volume set which is assigned main memory as the cache medium | STD, HIGH, VERY-HIGH | not satisfied (BY-CLOSE) |
Volume set whose volumes are connected to a cache controller | STD, HIGH | satisfied (IMMEDIATE) |
Table 20: Recommendations for performance profiles