PDS gas is the unreclaimed space in a PDS that was vacated when members were deleted or rewritten. Users often overallocate their PDSs to allow for the inevitable amount of PDS gas that would develop over time. With PDSEs, you do not need to add additional space to the allocation to allow for growth of the data set due to gas.

Studies show that a typical installation has 18% to 30% of its PDS space in the form of gas. This space is unusable to the data set until it has been compressed. A PDSE dynamically reuses all the allocated space according to a first-fit algorithm. You do not need to make any allowance for gas when you allocate space for a PDSE.

Space is only reclaimed for an OPEN for output when it is the only open for output on that system. PDSE space cannot be reclaimed immediately after a member is deleted or dated. If a deleted or updated member still has an existing connection from another task (or the input DCB from an ISPF edit session), the member space is not reclaimed until the connection is released and the data set is opened for output and that OPEN for OUTPUT is the only one on that system.

ABEND D37 can occur on a PDSE indicating it is FULL, but another member can still be saved in the data set. Recovery processing from an ABEND D37 in ISPF closes and reopens the data set. This new open of the data set allows PDSE code to reclaim space so a member can now be saved.