Constraint Control Statements › Constraint Descriptions › SIS: Setting Sequential Insert Strategy

SIS: Setting Sequential Insert Strategy

Normally, VSAM splits full CIs in half (at nearest logical record boundary) when required to make room for records that are inserted randomly into a KSDS file. This philosophy works well for truly random insertions.

Many applications randomly insert clusters of records instead of single records. These applications randomly position to a desired key in the file then insert several logical records in ascending key sequence, then randomly position and insert another cluster of records.

Since these files are opened with random access specified in the ACB, VSAM normally uses random insert strategy, which splits full CIs in half, as described above.

or an application that performs random clustered insertions, each CI split causes approximately one extra I/O for every (number of records in a CI)/2 whenever a CI split occurs. For example, if a CI holds ten records (average), then random insertion of these clustered records causes approximately one extra I/O for every five insertions. If many clustered inserts occur in the same control area, these inserts also cause CA splits, each of which causes potentially hundreds of I/Os.

Sequential insert strategy can avoid many of these CI and CA splits and the associated I/Os, if your inserts are mostly clustered, as described above.

SIS has no operands.