The DAB subsystem is the central BS2000 cache handler for the main memory cache medium.
A cache is a buffer for frequently used data. Accessing this buffer considerably speeds up disk I/Os. Unlike physical disk I/Os, the buffer is accessed in parallel, i.e. there is no need for disk-specific serialization by the
software/hardware.
DAB fulfills the following functions: configuring buffers (cache areas), providing information services for status inquiries, and handling I/Os with the cache. After creating cache areas for files or disk areas, DAB is involved in all
I/O operations for these files or disk areas in order to perform the desired buffering (caching).
The following are the main operating parameters of the cache areas managed by DAB:
selection of the data areas to be served
size of the buffer
caching mode (read cache, write cache, read/write cache)
size of the cache segments to be filled when data is cached
caching method (displacement according to LRU or no displacement, i.e. resident buffering)
with write caches, specification whether and when (i.e. at what cache occupancy level) write data in the cache is to be saved to disk.
“AutoDAB” includes functions for automatic and intelligent caching which carry out or facilitate correct selection of the above parameters. This greatly simplifies the administration of DAB cache areas while also improving cache memory utilization appreciably. If required, the operating parameters of such cache areas can also be set manually by systems support.
DAB functions are controlled using the DAB commands /START-DAB-CACHING, /MODIFY-DAB-CACHING, /SHOW-DAB-CACHING and /STOP-DAB-CACHING.
Output in S variables is supported for the output of information relating to the configuration of the cache areas.
The openSM2 software product can be used during DAB operation to monitor how efficiently it is functioning and to initiate any tuning measures that may be required.
DAB V21.0A can run under BS2000 V21.0A and higher on the supported servers.