Sphere and Splits in VSAM

A sphere is a base VSAM cluster and its associated clusters. These associated clusters are the alternate indexes (AIXs) of the base cluster.

CI and CA splits occur as a result of data record insertions (or increasing the length of an already existing record) in KSDS and VRRDS organizations. If a record is to be inserted (in key sequence) and there is not enough free space in the CI, the CI is split. Approximately half of the records in the CI are transferred to a free CI provided in the CA, and the record to be inserted is placed in the original CI.

If there are no free CIs in the CA and a record is to be inserted, a CA split occurs. Half of the CIs are sent to the first available CA at end of the data component. This movement creates free CIs in the original CA, then the record to be inserted causes a CI split.

As a result of the split, the physical sequence of records and CIs is no longer the same as the logical sequence. A new index entry is inserted in the sequence set for the new CI, and the existing index entry is updated.


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