For subtasking, I/O requests should be issued by the task that owns the DCB or a task that will remain active while the DCB is open. If the task that issued the I/O request ends, the storage used by its data areas (such as IOBs) can be freed, or queuing switches in the DCB work area can be left on, causing another task issuing an I/O request to the DCB to program check or to enter the wait state.

For example, if a subtask issues and completes a READ KU I/O request, the IOB created by the subtask is attached to the DCB update queue. (READ KU means the record retrieved is to be updated.) If that subtask ends, and subpool zero is not shared with the subtask owning the DCB, the IOB storage area is freed and the integrity of the ISAM update queue is destroyed. A request from another subtask, attempting to use that queue, could cause unpredictable abends. As another example, if a WRITE KEY NEW is in process when the subtask ends, a 'WRITE-KEY-NEW-IN-PROCESS' bit is left on. If another I/O request is issued to the DCB, the request is queued but cannot proceed.