Initial allocation or final release of a disk
During initial allocation of the disk concerned, a check is performed as to whether allocation and set cache attributes permit caching (command /START-DAB-CACHING ...,SHARED-DISK-SUPPORT=*YES/*NO; see "Caching for shared disks in the ADM-PFA concept").
When a disk is released (deallocated), all the released data is saved and invalidated in the cache. Invalidation ensures that no data of the disk concerned is read from the cache once the disk has been detached and reattached.
Allocation or release of disk space for a file
The data area served by DAB is automatically extended or reduced whenever disk space is allocated or released for a file served by DAB. This ensures that a file will be served by DAB to the full extent following storage space allocation or to the available extent following storage space reduction.
Furthermore, the associated cache area is extended if the data areas of an cache area are to be served using “resident buffering” (command /START-DAB-CACHING ...,CACHE-SIZE=*BY-FILE).
If, when processing data accesses or when adapting the DAB configuration to the modified storage requirements of a file, the system discovers that the DAB configuration is no longer up-to-date as a result of a previous system error and cannot be corrected using the information currently available, then the data located in the cache for the affected file is written back and this file is removed from the current cache configuration (see message NDB0145). The table shown below can be used to help resume caching of a file:
Cache class | Measures |
Cache areas with AutoDAB | None |
Cache areas with AREA=*FILE (...) | MODIFY-DAB-CACHING AREA=*ADD-FILE(...) |
|
|
Table 3: Measures for resuming file caching
Recataloging/deleting files served by DAB
DAB detects when a buffered file is renamed and updates the associated management data accordingly. Renamed files continue to be buffered by DAB.
If a temporary file is converted to a permanent one for cache areas with automatic caching (AutoDAB) in the main memory cache medium and CACHING-MODE=*BY-CACHE-MEDIUM, all write data of this file is saved if necessary. The file is subsequently only served with read caching.
The management data associated with any file that is deleted during DAB operation will be deleted in the corresponding cache area.
If a new file is created and assigned the name of the deleted file, it can be buffered in the previous cache area by using the command /MODIFY-DAB-CACHING ...,AREA=*ADD-FILE(<filename>). Alternatively, the file can be buffered in a new cache areas with an additional /START-DAB-CACHING command with AREA=*FILE.
In the case of a cache area with automatic caching, the next time it is opened it is reentered into the configuration of the existing cache area of the pubset or private disk.
When files buffered in an cache area with AREA=*FILE and CACHE-SIZE=*BY-FILE (resident buffering) are deleted, the cache area may be larger than required to serve the remaining files. It is up to systems support to check whether the space occupied by the cache is used economically and to reduce the cache area if required
(see the /MODIFY-DAB-CACHING command on "MODIFY-DAB-CACHING Modify parameters of DAB cache area dynamically").