VSE/VSAM performance: increased I/Os dependent on number of volumes in DEFINE CLUSTER
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VSE/VSAM performs increased I/Os (EXCPs) on the catalog volume dependent on the number of volumes specified in the DEFINE CLUSTER. The volumes do not need to be in use by the cluster, only specified as candidate volumes. Testing indicates that this begins to take place when the number of entries in the VOLUMES list for the cluster is above 27. This appears to apply to both the Data and Index component of a cluster.
Recommendation: The number of entries in the VOLUMES list for the cluster should stay below 28 to avoid performance degradation caused by increased I/O rates.