The control-interval (CI) size of SMF data sets can range from 0.5K (512 bytes) to 26K (26624 bytes) in size, with certain restrictions. The user specifies the CI size of the SMF data set and the device type when the VSAM data set is defined. Then, VSAM chooses the physical record size of the data set based on the specified CI size and the track size of the specified device type. SMF requires the CI size to equal the physical record size; otherwise, SMF cannot open the data set. Instead, SMF issues an error message to the console in addition to displaying the ‘feedback’ code.
The default CI size is 4K. Choose a larger value only if the default is so small that it makes the I/O rate too high.
When you choose a CI size, verify that the physical record size that VSAM selects for the DASD you specify equals that CI size. Table 1 shows some examples of valid user CI-size choices and the physical-record sizes that VSAM chooses for specific DASD.
| Control Interval (CI) Size | Physical Record Size | DASD |
|---|---|---|
| 16K (16384 bytes) | 16K (16384 bytes) | 3350 |
| 22K (22528 bytes) | 22K (22528 bytes) | 3380 |
| 26K (26624 bytes) | 26K (26624 bytes) | 3390 |
Before choosing a CI size, determine the size of your typical SMF record. To avoid wasting DASD storage, select a value that is either an integral multiple or an even divisor of that record size because records that cross a CI boundary when copied to DASD waste DASD storage. For example, if you specify a CI of 26K for a 30K SMF record, the record requires two 26K intervals (or 52K of storage) when copied to DASD, wasting 22K of DASD storage.
The CI size of the first SMF data set that the system opens during an IPL will be the CI size for all SMF data sets for that IPL. If SMF encounters a data set with a different CI size, then the data set is not used, and the system displays a message on the console informing the operator of the problem. A data set that has any error does not go on the list of active data sets. If all data sets fail to be successfully opened and allocated, then 4K (4096 bytes) is chosen as the default CI size for the IPL, and SMF buffers the data.
If you define a new group of SMF data sets with a different CI size, you must re-IPL to use these data sets. When you define an SMF data set, the logical record size must be 10 less than the CI size.