To allocate a PDS, specify PDS in the DSNTYPE parameter and the number of directory blocks in the SPACE parameter, in either the JCL or the data class. You must specify the number of the directory blocks, or the allocation fails.

If you do not specify a block size and the record format is fixed or variable, OPEN determines an optimum block size for you. Therefore, you do not need to perform calculations based on track length. When you allocate space for your data set, specify the average record length in kilobytes or megabytes by using the SPACE and AVGREC parameters, and have the system use the block size it calculated for your data set.

If your data set is large, or if you expect to update it extensively, it might be best to allocate a large data set. A PDS cannot occupy more than 65 535 tracks and cannot extend beyond one volume. If your data set is small or is seldom changed, let the system calculate the space requirements to avoid wasted space or wasted time used for recreating the data set.

VSAM, extended format, HFS, and PDSE data sets can occupy more than 65␠535 tracks.