The basic unit of data transfer between DASD and a buffer is a control interval (CI). At least one CI is read from or written to disk with each physical I/O operation. VSAM never transfers less than one CI. In certain instances, VSAM can transfer several CIs with a single I/O request by chaining several CCWs together into a single channel program. By reading or writing several CIs with a single I/O, fewer I/Os are required to process the data. This reduction in I/O results in improved performance.
Each CI usually contains more than one logical record. In this respect, CIs are similar to disk blocks that are used by non-VSAM access methods. Some CIs can contain hundreds of logical records, others can contain only a few records. Still other CIs can contain a single logical record, or even a portion of a logical record in the case of spanned records. A diagram of a control interval and its internal structure is shown below.
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