Data and index buffers are acquired and allocated only when the data set is opened. VSAM dynamically allocates buffers based on parameters in effect when the program opens the data set. Parameters that influence the buffer allocation are in the program's ACB: MACRF=(IN|OUT, SEQ|SKP, DIR), STRNO=n, BUFSP=n, BUFND=n, and BUFNI=n. Other parameters that influence buffer allocation are in the DD statement's AMP specification for BUFSP, BUFND, and BUFNI, and the BUFFERSPACE value in the data set's catalog record.

If you open a data set whose ACB includes MACRF=(SEQ,DIR), buffers are allocated according to the rules for sequential processing. If the RPL is modified later in the program, the buffers allocated when the data set was opened do not change.

Data and index buffer allocation (BUFND and BUFNI) can be specified only by the user with access to modify the ACB parameters, or through the AMP parameter of the DD statement. Any program can be assigned additional buffer space by modifying the data set's BUFFERSPACE value, or by specifying a larger BUFSP value with the AMP parameter in the data set's DD statement.

When a buffer's contents are written, the buffer's space is not released. The control interval remains in storage until overwritten with a new control interval; if your program refers to that control interval, VSAM does not have to reread it. VSAM checks to see if the desired control interval is in storage, when your program processes records in a limited key range, you might increase throughput by providing extra data buffers. Buffer space is released when the data set is closed.

Recommendation: Try to have data available just before it is to be used. If data is read into buffers too far ahead of its use in the program, it can be paged out. More data or index buffers than necessary might cause excessive paging or excessive internal processing. There is an optimum point at which more buffers will not help.