Phil, AFAIK, the whole GDG assignment only happens once.. during initial creation. MODing will change the size of the data set.. not the GooooVoo number(s). With SMS a new GDG entry gets rolled in at step end.
Most everything you need to know is in "Chapter 29. Processing Generation Data Groups" of the "DFSMS Using Data Sets" book. The link is http://pic.dhe.ibm.com/infocenter/zos/v1r13/index.jsp?topic=%2Fcom.ibm.zos.r13.idad400%2Fch14.htm The only thing I didn't see in there is a detailed explanation of the GDG roll-over when counting past 99999 generations. There are a couple of old APARS that go into great detail on how the process works and what to expect. Rob Schramm Rob Schramm Senior Systems Consultant Imperium Group On Fri, Oct 25, 2013 at 12:25 AM, Phil Smith <p...@voltage.com> wrote: > Apologies for the naïve question, but I spent some time Googling and > reading and didn't find a clear answer to this. > > We have a data set that gets fetched from the network and changes > occasionally. I'd like to untie this update from normal operation-that is, > on the off-chance that the network is down at the precise instant that we > check for the data set, I'd like things to continue operating. So I'm > thinking that if we cache a copy of the data set on disk (the contents > aren't sensitive), we could try to fetch; if we can't fetch, we shrug and > continue; if we can, we compare the data read with the existing data > (there's a timestamp) and only rewrite if it's changed. > > But the data is also shared across tasks, so we don't want a window where > it's half-written and some task tries to read it. > > A GDG seems like it would work great for this: the number of generations > could be defined as low as two, so the update would write the "other" copy, > and the next time one of the tasks tries to read it, it would get the newer > one. > > So here's my question (aside from the obvious one of "Am I missing > something above that makes this dumb"): > > 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". > > Thanks, > -- > ...phsiii > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN > ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
