Determining the size of CIs and blocks

You can specify the CI size for your database. The DBDGEN utility calculates the CI size for you, and you must use this size in your DEFINE CLUSTER command when you use IDCAMS to create the data set.

Based on CI size, VSAM determines the size of physical blocks on a DASD track. VSAM always uses the largest possible physical block size, because the largest block size best utilizes space on the track. So your choice of a CI size is an important one. Your goal in picking it is to keep a high percentage of space on the track for your data, rather than for device overhead.

Track sizes vary from one device to another, and many different CI sizes you can specify exist. Because you can specify different CI sizes, the physical block size that VSAM picks varies and is based on device overhead factors. For information about using VSAM data sets, refer to z/OS® DFSMS Access Method Services for Catalogs.