The ALLOCATE option applies only to unordered data sets. This option controls the allocation strategy used to obtain space for new records.

The records in an unordered data set are not maintained in logical order. Records are simply grouped in blocks, and these blocks are linked together. For disjoint data sets, there is one chain of blocks. For embedded data sets, there is one chain for each master and each chain contains records for a single master.

No free space table is kept; rather, the available record spaces within each block are linked together. If the DENSELY keyword is specified, the system searches for an available record space in any one of the allocated blocks before allocating a new block. If the FROM END keyword is specified, the system examines only the first and last blocks of the chain before it allocates a new block. The FROM END keyword prevents available space in intermediate blocks from being reused. Use of the FROM END keyword can reduce the time required to add records to an unordered data set, especially during the initial load of the database. The default value is DENSELY.