A PDS is stored only on a direct access storage device. It is divided into sequentially organized members, each described by one or more directory entries.

Each member has a unique name, 1 to 8 characters long, stored in a directory that is part of the data set. The records of a given member are written or retrieved sequentially.

The main advantage of using a PDS is that, without searching the entire data set, you can retrieve any individual member after the data set is opened. For example, in a program library that is always a PDS, each member is a separate program or subroutine. The individual members can be added or deleted as required. When a member is deleted, the member name is removed from the directory, but the space used by the member cannot be reused until the data set is reorganized; that is, compressed using the IEBCOPY utility.

The directory, a series of 256-byte records at the beginning of the data set, contains an entry for each member. Each directory entry contains the member name and the starting location of the member within the data set (see Figure 1). You can also specify as many as 62 bytes of information in the entry. The directory entries are arranged by name in alphanumeric collating sequence.

Related reading: See z/OS DFSMS Macro Instructions for Data Sets for the macros used with PDSs.

Figure 1. A Partitioned Data Set (PDS)

The starting location of each member is recorded by the system as a relative track address (from the beginning of the data set) rather than as an absolute track address. Thus, an entire data set that has been compressed can be moved without changing the relative track addresses in the directory. The data set can be considered as one continuous set of tracks regardless of where the space was actually allocated.

If there is not sufficient space available in the directory for an additional entry, or not enough space available within the data set for an additional member, or no room on the volume for additional extents, no new members can be stored. A directory cannot be extended and a PDS cannot cross a volume boundary.