At 21:25 -0700 on 10/24/2013, Phil Smith wrote about GDG question:
If a jobstep runs with a DD for a GDG that's got DISP=MOD, and the jobstep reads but never tries to write the file, does a new, zero-length GDG get created? I'm of course hoping the answer is "No".
The only time a new GDG dataset is created is when you use a DSN of GDG(+x) [GDG(-x) are older generations]. In your case you are asking for GDG(0) so you can update it without creating a new generation. Note however that DISP=MOD is the same as DISP=OLD so far as that jobstep having exclusive access to the dataset (so no other job step can read it). The difference between DISP=OLD and DISP=MOD for an existent dataset is writes to the former erase the data (ie: Only the data you write will be there) while the later leaves the old data and adds the new to the end of the dataset.
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