In the VSAM Control Interval diagram, the fields marked LRx represent Logical Records contained in the control interval. The fields marked RDFx represent Record Definition Fields. The last field, marked CIDF, represents the Control Interval Definition Field. The shaded area marked FREESPACE is space that is currently unused in this control interval.
The RDFs and CIDF are VSAM control fields that are contained in almost every control interval. These fields are wasted space in the sense that they contain no useful application data. However, these fields contain critical VSAM information, and are required.
Note: RDFs exist only in CIs that contain logical records. An empty CI does not contain any RDFs. Even empty CIs contain a CIDF, except CIs within a Linear Data Set (LDS). LDS clusters contain only application data; there are no CIDFs or RDFs in any LDS CI.
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