gtpa2m3p | Application Programming |
You may have some data that is highly accessed, but only temporarily or intermittently. In this case, you would like to have fast access (requiring no I/O) to the data when it is needed, but you are reluctant to allocate the data to main storage permanently.
The virtual file access (VFA) facility temporarily retains both program and record data that has been retrieved from file storage in a specially allocated area of main storage. A file reference request searches the VFA area for the data to be accessed; if it is not found, then the search moves to file storage, but if it is found, the relatively slow I/O operation to file storage is avoided. Note that if the data is found in the VFA area, then any wait system service request to await the completion of I/O does not reset the 500-millisecond timeout counter.
Data remains in the VFA area until the VFA area is full; then the least used data is filed and removed from the VFA area to make room for more frequently used data. In this way, data that is occasionally highly accessed (such as records that are updated every hour) remains in main storage during high activity, but can be filed to file storage when activity slows.