If the number of I/O buffers provided for index records is greater than the number of requests that require concurrent positioning (STRNO), one buffer is used for the highest-level index record. Any additional buffers are used, as required, for other index-set index records. With direct access, you should provide at least enough index buffers to be equal to the value of the STRNO parameter of the ACB, plus one if you want VSAM to keep the highest-level index record always resident.

Unused index buffers do not degrade performance, so you should always specify an adequate number. For optimum performance, the number of index buffers should at least equal the number of high-level index set control intervals plus one per string to contain the entire high-level index set and one sequence set control interval per string in virtual storage.

VSAM reads index buffers one at a time, and if you use shared resources, keep your entire index set in storage. Index buffers are loaded when the index is referred to. When many index buffers are provided, index buffers are not reused until a requested index control interval is not in storage. Note that additional index buffers is not used for more than one sequence set buffer per string unless shared resource pools are used. For large data sets, specify the number of index buffers equal to the number of index levels.

VSAM keeps as many index-set records as the buffer space allows in virtual storage. Ideally, the index would be small enough to permit the entire index set to remain in virtual storage. Because the characteristics of the data set cannot allow a small index, you should be aware of how an index I/O buffers is used to determine how many to provide.