z/OS V1R6.0 DFSMS Access Method Services for Catalogs

DEFINE CLUSTERThe size of the control interval depends on the maximum size of the datarecords and the amount of buffer space you provide.LSR/GSR buffering technique users can ensure buffer pool selection byexplicitly defining data and index control interval sizes.If CONTROLINTERVALSIZE is not coded, VSAM determines the size ofcontrol intervals. VSAM selects a control interval size for the data componentthat will optimize direct access storage usage. It will then select an indexcontrol interval size based on the number of data control intervals in the datacontrol area.sizeIndicates a cluster’s data and index component size..If SPANNED is not used, the size of a data control interval must be at least7 bytes larger than the maximum record length.If the control interval specified is less than maximum record length plus a7-byte overhead, VSAM increases the data control interval size to containthe maximum record length plus the needed overhead.If SPANNED is specified, the control interval size can be less than themaximum record length. You can select a size from 512 to 8K in incrementsof 512, or from 8K to 32K in increments of 2K. When you choose a sizethat is not a multiple of 512 or 2048, VSAM chooses the next highermultiple. For a linear data set, the size specified is rounded up to 4096 ifspecified as 4096 or less. It is rounded to the next higher multiple of 4096 ifspecified as greater than 4096.The size of the index control interval is the number of data control intervals ina data control area that need indexing at the sequence set level of the indexcomponent. The size of each entry depends on an average compression valuefor a user key. The keys will compress to 1/3 of the length of the actual keyvalue. In some cases, the general compressed key length on which thealgorithm is based will be affected by the actual values and ordering of theuser key. The result is that each entry can occupy more space in the indexrecord than that provided. This may result in additional control area splits andin all cases, wasted space in the data set. If after loading the data sets, thiscondition exists; noted by more than anticipated space to store the data set onthe direct access device. You should increase the index control interval size.The size can be increased incrementally until it is felt that this condition nolonger exists. The guideline formula documented in the past is as follows:(KEYLEN/2 ) * DATA CI/CA less than or equal to INDEX CISIZE.You should be aware that this is only a guideline and does not take intoaccount the actual algorithm for determining the index control interval sizerequirement. However, the 2:1 compression of key length in the above formulaprovides some additional overhead over the actual 3:1 formula used during theactual algorithm. Using the above formula can result in an index controlinterval size that is too large. This may increase I/O transfer time for eachindex component record, or it may be too small to address the conditiondescribed above.Refer to “Optimizing VSAM Performance” in z/OS DFSMS: Using Data Sets fora discussion of control interval size and physical block size.Abbreviations: CISZ or CNVSZChapter 14. DEFINE CLUSTER 153