KSDS files contain two types of control intervals. The first type of control interval is the data control interval, which contains the logical records and VSAM control information as shown in the VSAM Control Interval diagram in the section Buffering Basics. The other type of CI is the index control interval. The index CIs contain index entries which point to other index CIs or to data CIs. These indexes allow VSAM to retrieve records randomly by a field in the record which is called a key. VSAM maintains index CIs transparently on behalf of the application programs.
All data CIs for a particular cluster are the same size. All index CIs for a particular cluster are also the same size, although the data control intervals for a particular cluster can be a different size than the index control intervals for the same cluster. For example, a particular KSDS can be defined with 4K data CIs and 1K index CIs.
Each virtual storage buffer is the same size as the control interval for which it is built. For example, a KSDS file with an index CI size of 1K and a data CI size of 4K requires some number of 1K buffers for index control interval I/O, and some number of 4K buffers to be used exclusively for data CI I/O.
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