DAB uses the main memory (MM) as cache medium.
Main memory has the following advantages compared to external memory media:
fast access times
no significant wait times between the start and end of a data transfer and consequently no system loading from task changing in this phase
no system loading from interrupt handling at the end of a data transfer
The administration data required by DAB in the main memory resides in resident data spaces of the Enterprise System Architecture (ESA). The number of these data spaces is limited to 1024, but because of other users (components of the operating system) fewer data spaces may be available. This will determine the maximum number of DAB cache areas.
The DAB subsystem permits main memory areas of any BS2000 system to be used for caching.
Main memory is best suited as a read cache since it is a volatile cache medium and data stored in it is no longer accessible after a system crash. If it is to be used as a write or read/write cache, this type of use is recommended for the following:
files requiring no higher failsafe level during processing
(temporary work files, SYSEAM)files whose write data is protected by an additional mechanism (e.g. logging)
files that can be fully restored without excessive effort after a system crash.
Main memory cache areas can exceed 2 GB; their maximum size is currently only restricted by the physical memory available.
DAB requires at least 2 data spaces (for cache administration data) per cache area in the main memory, the maximum number of main memory cache areas is therefore 512 (if there are no other users of resident data spaces in the system).
Information on performance is provided in section “CPU utilization”.