Read caching with AREA=*FILE (i.e. without automatic caching)
Caching data areas of this type with DAB is critical unless all systems involved only read the data. Otherwise, such data areas should not be served by DAB, not even in read caching mode.
The SHARED-DISK-SUPPORT operand of the /START-DAB-CACHING command can be used to define whether such shared data areas are to be buffered or not (*NO is the default).
When allocating a disk, DAB adjusts its current allocation state of the cache areas (SHARED/EXCLUSIV) with the SHARED-DISK-SUPPORT attribute. Areas of a shared volume are only served if the corresponding cache area was created with SHARED-DISK-SUPPORT=*YES. The F5 and F1 labels are never served.
Read caching with AREA=*BY-SYSTEM
Data consistency when read caching shared volumes is guaranteed by DAB. Systems support does not have to check to ensure that the served files are only read by all systems.
In this case, files that are only being used locally at the time of processing are cached. Data consistency is ensured by DMS on the one hand since it prevents the files from being opened by another computer during processing. On the other hand, DAB also ensures data consistency with the following measures:
When a file is closed, all copies of it in the cache are invalidated so that any subsequent (read) accesses to this file must always be to the disk. This ensures that the latest data is always processed, even after an (intermediate) data update by another system.
The SHARED-DISK-SUPPORT operand is meaningless in this case and is ignored.
(Read/)write caching
In addition to the explicit setting of write or read/write caching, it is also possible to set read/write caching using CACHING-MODE=*BY-CACHE-MEDIUM, depending on the cache medium and the data area specification.
Caching shared data areas in write or read/write mode is not possible and is prevented by DAB. The SHARED-DISK-SUPPORT operand is not processed in this case and an appropriate message is output.
Data areas on shared pubsets are not served in these modes.