When a PDSE member is updated or replaced, it is written in the first available space. This is either at the end of the data set or in a space in the middle of the data set marked for reuse. This space need not be contiguous. The objective of the space reuse algorithm is not to extend the data set unnecessarily.

With the exception of UPDATE, a member is never immediately written back to its original space. The old data in this space is available to programs that had a connection to the member before it was rewritten. The space is marked for reuse only when all connections to the old data are dropped. However, once they are dropped, there are no pointers to the old data, so no program can access it. A connection may be established at the time a PDSE is opened, or by BLDL, FIND, POINT, or DESERV. These connections remain in effect until the program closes the PDSE or the connections are explicitly released by issuing DESERV FUNC=RELEASE, STOW disconnect, or (under certain cases) another POINT or FIND. Pointing to the directory can also release connections. Connections are dropped when the data set is closed.

Related reading: For more information about connections see z/OS DFSMS Macro Instructions for Data Sets.