Compressed data records in an extended-format key-sequenced data set have a different format than noncompressed data records. This format includes a record prefix that contains internal compression information. When the record is a spanned record, each segment of the record contains a segment prefix with information similar to the record prefix for describing the segment. The length of the record prefix for nonspanned records is 3 bytes, and the length for spanned records is 5 bytes.

The stored record format has no affect on the data seen by the user as a result of a VSAM GET request. In addition, no special processing is required to place the record in the data set in a compressed format.

The presence of the record prefix does result in several incompatibilities that can affect the definition of the key-sequenced data set or access to the records in the key-sequenced data set. When a VSAM data set is in compressed format, VSAM must be used to extract and expand each record to obtain data that is usable. If a method other than VSAM is used to process a compressed data set and the method does not recognize the record prefix, the end result is unpredictable and could result in loss of data. See Compressed Data.