This week I will tell you about some tricks with the SPACE parameter used with DD-cards in JCL in combination with DISP=SHR (or DISP=OLD). Though it does seem silly, it is indeed possible to use the SPACE parameter with existing datasets and DISP=SHR. You primarily use it to tell JES what secondary amount of space you want allocated. For example you can write SPACE=(TRK,(1,999)) to make a secondary allocation of 999 tracks.

The point is that if the dataset extents while running with a SPACE parameter JES will use the secondary allocation from the SPACE parameter instead of the original one registered for the dataset. In other words you can temporarily override the normal secondary allocation with a much bigger one. If the dataset originally were allocated without secondary extents, it is able to get one.

I have never tried it, but I am sure that if you write SPACE=(TRK,(1,0)) you will be unable to take further extents, even though the dataset is created with secondary extents. It is worth notifying, that the RLSE parameter will be honoured as well, but only if the dataset is opened for output. Unfortunately IEFBR14 does not open for output.