VSAM data set design parameters

D. G. Keehn, J. O. Lacy
1974 IBM Systems Journal  
A general description of the Virtual Storage Access Method ( V S A M ) is followed b y a qualitative discussion of performance expectations. V S A M data-set design parameters are discussed with respect to performance tradeofs. Analytic techniques are developed for relating some of the VSAM performance sensitivities to data set design parameters. 186 KEEHN AND LACY IBM SYST J index A VSAM Key Sequenced Data Set (KSDS) has an index for direct operations. Many index entries are stored together on
more » ... DASD, and they are accessed in blocked units. Each index record is a key-pointer pair where they key is the highest key in the pointed-to block, and where the pointed-to block is another index block or Control Interval ( C I N V ) in the data set. The blocks make up levels where the highest level consists of a single block, and each of the lower levels consists of the blocks pointed to by the next higher level. This may be thought of as a fan-out effect. Each of the blocks in the lowest level of the index (called the Sequence Set) addresses a particular Control Area (cA), and resolves a key to the C I N V in which the desired record resides (if it exists). Figure 2 represents a fan-out, in tree form for a three-level VSAM index. A record is located via this index tree by using a less-than-orequal comparison on each level until the appropriate C I N V is located. Optionally, the Sequence Set can be stored with the data it addresses. If this is the case, the Sequence set occupies the first track of the C A , and is replicated as many times as will fit on the track (to save rotational delaylatencywaiting for the beginning of the desired record to come under the read/write head). Higher levels of the index may also be replicated. VSAM compresses'the key entries in the index so that redundant key 188 KEEHN A N D LACY IBM SYST J
doi:10.1147/sj.133.0186 fatcat:gg7mfnykgbavxi3tch4zoefv7e