Sharing within a Computer System

Specifying DISP=OLD, NEW, or MOD restricts access to a PDSE by a single job, started task, or TSO/E user. DISP=SHR lets multiple jobs or users access the PDSE at the same time, and permits sharing between DCBs in the same job step and in different jobs.

Member-Level Sharing. As in a PDS, multiple copies (versions) of a member having the same name but different TTRs can exist in a PDSE at the same time. Sharing is done on a "per version" level. The sharing rules depend on whether the DCB is open for input, update, or output.

INPUT—A version of a member can be accessed by any number of DCBs open for input.

UPDATE—You cannot have any DCBs reading a version of a member while another DCB is updating the same version of the member.

OUTPUT—Any number of DCBs open for output can create members at the same time. The members are created in separate areas of the PDSE. If the members being created have the same name (specified in the STOW done after the data is written), the last version stowed is the version that is seen by users, and the storage occupied by the first version is added to the available space for the PDSE. You can have:

Multiple DCBs reading and creating new versions of the same member at the same time. Readers continue to see the "old" version until they do a new BLDL or FIND by name.
A single DCB updating a version of a member while multiple DCBs are creating new versions of the member. The user updating the data set continues to access the "old" version until the application does a new BLDL or FIND by name.