Notes concerning DAB usage

The selection of the database to be buffered and the prefetching factor setting should be done via automatic caching. It is therefore advisable to serve performance-critical data storage media with AutoDAB. This is done ideally with a single /START-DAB-CACHING command with which the volumes concerned are specified and all available cache memory is used.

The SM2 software product (see “openSM2” manual [7 (Related publications)]) can be used to monitor DAB efficiency during operation and to initiate any tuning measures that may be required.

The following points are relevant when using DAB:

1. Write caching is not allowed for any files of the home pubset accessed by BS2000 before ’System Ready’ (e.g. TSOSCAT, JOIN or SYSSRPM files, REP files etc.).

2. When caching volumes with high availability (DRV, RAID1, RAIDS) it must be ensured that availability is not reduced by using the cache. Pure read caches are uncritical in all cases.

3. DAB cannot be used to buffer the paging area.

4. For data security reasons, write caching of file catalogs (TSOSCAT) does not make use, even if the associated cache area has been activated in write mode.

5. When a disk served by DAB is detached by means of the command /DETACH-DEVICE ...,FORCE=*YES, any data on this disk that may still be contained in the cache will not be saved. However, the buffered disk data continues to be held in the cache and will be available again with the next allocation of the disk on the same system.

   Reattaching the disk to another system is likely to result in data loss since cache areas of this type cannot be reassigned to another system. In addition, if the disk is subsequently reattached to the original system again, any cache areas on this computer that serve data areas of the disk must first be released by means of the /FORCE-STOP-DAB-CACHING command to prevent any data on this disk that may still be held in the cache from being used.

6. All assigned cache areas must be terminated before formatting disks (VOLIN utility) or regenerating pubsets (SIR utility).

7. Since the cache areas that can be created can be very large, more time must be allowed for saving the cache data when releasing large cache areas used in read or read/write mode.

8. Refer to section "DAB in multiprocessor operation" for notes on caching with shared pubsets.

9. Cache areas are created as resident storage when the operand MEMORY=*STD/*BELOW-MIN-MEM-SIZE is specified in the START-DAB-CACHING command. It must be ensured that the main memory is adequately dimensioned according to the cache size.
In the case of operation on a virtual machine, its MINIMAL-MEMORY-SIZE must also be dimensioned appropriately.

10. If, in a cache area using automatic caching, cache overflows occur over several monitoring periods or too many files have to be excluded from caching due to their poor cache utilization, systems support is informed via console message NDB0109 that the cache area is too small for the current load. The cache area should be increased in size with the /MODIFY-DAB-CACHING command in this case.

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