gtpd1m0vDatabase Reference

Virtual File Access (VFA)

Virtual file access (VFA) provides an intermediate staging area between the application program data and the direct access file database. Because the VFA area is in real main storage, much higher access rates can be achieved and input/output (I/O) channel load reduced.

VFA is intended to effectively use the real main storage that cannot be efficiently used by TPF working storage and main storage resident application programs. VFA is also used by the TPF system as an intermediate file storage area to avoid having to maintain information in an entry control block (ECB) about when a file completed. When an application issues a file, the data record is copied into VFA whether or not it is a VFA candidate, unless it is a 4 KB non-candidate record. If the record is not a candidate then the record is filed from VFA by the control program and removed from VFA. If the record is a VFA candidate, then the record is handled according to the rules described below for VFA candidates.

VFA is conceptually similar to the virtual storage page pools used in IBM virtual storage operating systems. Since the number of records competing for residency in VFA storage greatly exceeds the amount of VFA storage available, only the most active records will be readily accessible through VFA. However, the design does allow for some selectivity in the number and type of records that can become resident in VFA so all of the database does not have to compete for VFA storage.

To further relieve I/O channel load, VFA permits multiple updates to be applied to a record resident in VFA without the updates being reflected to the database copy until necessary.