One or more SF pubsets or SM pubset volume sets can be assigned to a cache medium. The MODIFY-PUBSET-CACHE-ATTRIBUTES (EDIT-PUBSET-CACHE-ATTRIBUTES) command enables systems support to specify, among other details, the proportion of the cache in each cache medium which is to be reserved for a pubset. This assignment automatically takes effect when the pubset is imported (IMPORT-PUBSET), or when a volume set is activated for an SM pubset (MODIFY-PUBSET-PROCESSING), but can also be activated dynamically while the pubset is in use, see "Starting and terminating caching dynamically".
Details of the cache assignment of pubsets are provided in section "Defining the cache configuration" and in section “Modifying the cache configuration” in chapter "Modifying dynamic configuration data of SF and SM pubsets".
Systems support can gain information on cache hit rates from the analysis of the PFA openSM2 monitoring report, which provides information about the use of the cache media.
The SHOW-PUBSET-CACHE-ATTRIBUTES command enables systems support to obtain information about the cache assignments which are currently valid and activated, about the cache media, their operating modes and the size declarations. Detailed information on the configuration of the separate PFA cache areas can additionally be obtained with the SHOW-CACHE-CONFIGURATION command.
Setting cache usage globally
Systems support can permit one-off cache use for all the files on a pubset and all users by specifying CACHED-FILES=*ALL in the MODIFY-PUBSET-CACHE-ATTRIBUTES command. If main memory is used as a cache medium, it is also possible to specify CACHED-FILES=*BY-SYSTEM, in which case the cache-relevant files are determined by the system itself, which also monitors their cache utilization. This ensures optimum performance for a pubset’s files (so-called AutoDAB).
Here default values are first set for each of the pubset’s files (with the exception of some system files, e.g. file and user catalog):
“Read cache” for the volatile cache medium main memory
With the data area specification AREA=*BY-SYSTEM (START-DAB-CACHING command): for temporary files in main memory, also “read-write cache”
Users can modify this presetting without requiring any additional authorization.
Alternatively, certain users can be permitted to use caching by allowing them to assign their file attributes.
Setting cache usage on a user-specific basis
The task of systems support is to authorize user IDs to use caches in processing their files (hiperfiles) and additionally for SM pubsets, to assign the user sufficiently sized quotas for hiperfiles (HIGH-PERF-SPACE and VERY-HIGH-PERF-SPACE, see "Defining user quotas").
Each authorization is issued by making an appropriate entry in the user catalog for the pubset, using the command ADD-USER or MODIFY-USER (DMS-TUNING-RESOURCES operand); this indicates the highest permissible performance attribute for the user’s files. The MODIFY-PUBSET-USER-ATTRIBUTES command is additionally available for assigning the required quotas for SM pubsets.
Systems support can issue the following caching authorizations to each individual user:
NONE
The user is given no authorization to use a cache in processing files.CONCURRENT-USE
The user is authorized to use a cache in processing files, but in doing so competes with all other users who have the same authorization. This means that if there is a shortage of storage space in the cache medium, sections of the user’s file may again be stored elsewhere.EXCLUSIVE-USE
The user is authorized to process files exclusively using a cache. Even when there is a shortage of storage space, the system will always attempt to hold the hold the user’s file entirely in the cache medium, if the user has requested this by an appropriate file attribute.
Systems support and the user can obtain information about the pubset-specific authorizations and quotas for a particular user ID by means of the SHOW-USER-ATTRIBUTES command.
The home pubset cannot be buffered in the cache media managed by DAB by means PFA caching. In the event of a system failure, it may not be possible to reconstruct the write data because the DAB subsystem is not available at the time the home pubset is imported. An attempt to set up a PFA cache area for the home pubset is therefore rejected.
Starting and terminating caching dynamically
The command START-FILE-CACHING can be used to start caching for files which are already open. The PERFORMANCE and USAGE operands are used to determine the file’s performance value and cache mode attributes.
Preconditions for the enabling of caching for files which are already open are
the caller must have access authorization to the file (file owner or systems support)
the pubset on which the file is located
must possess a valid, activated cache assignment
must be locally accessible
the command must be entered at the same system of the cache that is assigned to the pubset which is to be operated on the local system
the caller must have been granted the necessary cache authorization by systems support
If the file is not open or if it is already being processed with caching, then START-FILE-CACHING is rejected.
The SHOW-DAB-CACHING command informs systems support about the currently installed cache areas in the cache media main memory.
The command STOP-FILE-CACHING terminates caching for an open file or a file for which data is still present in the cache. The data still present in the cache is written back (exception: read-only cache) and the contents of the cache are invalidated. The same preconditions must be satisfied as in the case of START-FILE-CACHING.