If a PFS communicates with a separate address space, that is, one unknown to z/OS UNIX, and waits for replies from that address space, users could be left waiting forever if that address space abnormally terminates while it has outstanding responsibilities to post user threads. Usually, the PFS has to remember all users that are waiting in this situation and post them from a recovery resource manager of the separate address space. This can involve extra serialization and overhead during mainline operations.

If, however, the separate address space registers with the v_reg() function, specifying the PFS that is dependent on it, and uses osi_wait and osi_post, the system remembers this information in a task-related area that does not require additional serialization or overhead during mainline operations. When the separate address space terminates, the system scans through all users looking for those in a potential wait for this address space and posts them. Thus the extra overhead is incurred only when the separate address space terminates.