Introduction › VSAM Buffering and Performance › Application Interface

Application Interface

The application program is usually interested in logical records and not control intervals. VSAM manages the control intervals on behalf of the application by:

• Finding the control interval that contains the requested logical record. For keyed requests against a KSDS file, VSAM first searches the index. This processing usually requires at least one I/O operation, and sometimes multiple I/Os. These I/Os use the buffers that were allocated for the index CIs.
• Retrieving the data control interval. The preceding step found where the record resides; now VSAM must actually read that data CI. This processing usually requires at least one I/O operation, and can require several I/Os in the case of spanned records. These I/Os use the data buffers.
• Deblocking the requested logical record. Applications are usually written to handle a single logical record at a time - not an entire CI. Once VSAM reads the entire data CI into a buffer, VSAM then finds the desired logical record and extracts just that record for the application.
• Passing the logical record back to the application or inserting the logical record into the control interval for a write request. Once the record is located, it is usually moved to an application program work area (move mode processing).

Note: The first and second items can require several I/Os to the DASD. Some of these I/Os can be avoided if the required control intervals are already resident in virtual storage buffers.