A relative record data set has records that are identified by their relative record number (RRN). The first record in the data set is RRN 1, the second is RRN 2, and so on.
Records in an RRDS can be fixed or variable length records, and the way in which VSAM handles the data depends on whether the data set is a fixed or variable RRDS.
A fixed RRDS has fixed-length slots predefined to VSAM, into which records are stored. The length of a record on a fixed RRDS is always equal to the size of the slot.
Fixed Length vs Variable Length.
Fixed length RRDS
VSAM locates records in a fixed RRDS by multiplying the slot size by the RRN (which you supply on the file control request), to calculate the byte offset from the start of the data set. A fixed RRDS generally offers better performance.
A variable RRDS, can accept records of any length up to the maximum for the data set. In a variable RRDS VSAM locates the records with an index. A variable RRDS offers greater function.
With releases of DFSMS/MVS 1.5 and later, a data set can be greater than 4 GB in size if it is defined as extended format and extended addressability in the storage class.
CICS supports access to extended RRDS or VRRDS data sets if you use an RRN that can be specified in a four byte RRN field to access the records that reside beyond the 4 GB boundary.