Write caching

The file access times for write requests can be shortened considerably by using either the write caching or
write/read caching mode.

DAB accelerates any write access processing, irrespective of data locality, provided write caching or read/write caching is enabled and free segments are available. Caching ensures that the data is written from cache to disk asynchronously from the I/O job. With good locality, this significantly contributes to reducing the I/O system load.

Write caching offers the following advantages:

• From the point of view of the requesting programs, the file access times can be shortened considerably since the data to be transferred is first moved to the cache. As far as the application is concerned, the write request has been completed. The user can specify during cache area definition or in ongoing operation (/START-DAB-CACHING, /MODIFY-DAB-CACHING commands, FORCE-OUT operand) at which point the transfer occurs. The transfer of the buffered data from cache to disk does not normally affect program runtime.

• The I/O system load is reduced:
  In the event of good spatial or temporal locality of the buffered data, buffering and asynchronous writing of the data to disk results in the number of disk accesses actually performed being smaller than the total number of data transfers initiated by the programs.
  With a suitably large cache, the transfer of buffered data from cache to disk before the application is closed can be omitted entirely, which minimizes the I/O system load. Subject to good temporal locality of the buffered data, read accesses following the transfer of the data to the cache can be satisfied from the cache efficiently.

Using the caching technique “resident buffering” effectively precludes cache overflows, provided the cache size matches the size of the area it serves, i.e. a sufficiently large cache area must be created. It must be noted, that the cache area can grow and thus free cache space must be made available.