When you request a control interval size, you must consider the length of your records and whether the SPANNED parameter has been specified.

The valid control interval sizes and block sizes for the data or index component are from 512 to 8192 bytes in increments of 512 bytes, and from 8 KB to 32 KB in increments of 2 KB. When you choose a CI size that is not a multiple of 512 or 2048, VSAM chooses the next higher multiple. For a linear data set, the size specified is rounded up to 4096 if specified as 4096 or less. It is rounded to the next higher multiple of 4096 if specified as greater than 4096.

Example: 2050 is increased to 2560.

The block size of the index component is always equal to the control interval size. However, the block size for the data component and index components might differ.

Example: Valid control interval sizes are 512, 1024, 1536, 2048, 3584, 4096, ... 8192, 10␠240, and 12␠288, and so on.

Related reading: For more information, see the description of the CONTROLINTERVALSIZE parameter of the DEFINE CLUSTER command in z/OS DFSMS Access Method Services Commands.

Unless the data set was defined with the SPANNED attribute, the control interval must be large enough to hold a data record of the maximum size specified in the RECORDSIZE parameter. Because the minimum amount of control information in a control interval is 7 bytes, a control interval is normally at least 7 bytes larger than the largest record in the component. For compressed data sets, a control interval is at least 10 bytes larger than the largest record after it is compressed. This allows for the control information and record prefix. Since the length of a particular record is hard to predict and since the records might not compress, it is best to assume that the largest record is not compressed. If the control interval size you specify is not large enough to hold the maximum size record, VSAM increases the control interval size to a multiple of the minimum physical block size. The control interval size VSAM provides is large enough to contain the record plus the overhead.

For a variable-length RRDS, a control interval is at least 11 bytes larger than the largest record.

The use of the SPANNED parameter removes this constraint by permitting data records to be continued across control intervals. The maximum record size is then equal to the number of control intervals per control area multiplied by control interval size minus 10. The use of the SPANNED parameter places certain restrictions on the processing options that can be used with a data set. For example, records of a data set with the SPANNED parameter cannot be read or written in locate mode. For more information about spanned records see Spanned Records.