If your storage control unit supports the caching of write requests, you can use the SETCACHE line operator to establish or terminate the DASD FAST WRITE and CACHE FAST WRITE functions. Read and system caching must be active in order for fast write (both DASD and CACHE) to be in effect.

DASD FAST WRITE provides support for the caching of write requests. NON-VOLATILE STORAGE (NVS) provides backup storage for this function. When you write data out to a volume, a write request is made to the storage control unit. The storage control unit accepts the data and puts it in the storage control unit cache. Later, the storage control unit schedules the data for writing.

When the data is stored in the storage control unit cache, it is also written to NVS. NVS is a buffer backed up by battery power that can maintain data up to 48 hours. NVS provides insurance against the loss of data written to the storage control unit cache and not yet sent to DASD.

The CACHE FAST WRITE feature allows part of the subsystem storage to be set aside as work space. Data written out in CACHE FAST WRITE mode is not staged for writing to DASD unless you make an explicit request or the system needs to reuse some of the cache space. CACHE FAST WRITE is for temporary work files only because data in cache is eventually discarded.