When DISP=MOD is used and there is existing data on the data set I think there are three possibilities (in likelihood order) with the existing data: 1) short or coincidentally-full block, unfilled track; 2) short block, otherwise-coincidentally-filled track; 3) full block, filled track (double coincidence).
For 1), the first block written for "MODding" will be the first block on the "next" track, with the remainder of the final track of the previous content of the data set being "empty", screwing up a "seek" (there will obviously be a short block in one case, and a full one in the other). For 2), the first block written for "MODding" will be the first block on the "next" track, and the previous block will be short, screwing up a "seek". For 3), by the double coincidence, everything will be fine, and there will be no evidence from the content of the data set itself that it was ever MODded. "seek" works perfectly. May not, or may, on the next run. It is unproblematic for "seek" if the final block is short, or the final track is short, or both. But, entirely problematic - at least potentially - for subsequent data (that which has been MODded) if there is any "gap" of any type (short block, short track), if there are embedded short blocks or unfilled tracks. Whether the "seek" would work sometimes, fail all the time, or the programmer noted something but did not diagnose but coded-around, and probably other scenarios, will be down to the actual data and program. For ordinary sequential reads, nothing with DISP=MOD is problematic (assuming that RECFM/LRECL are consistent with how later used). On Fri, 6 Jan 2017 14:55:22 +0000, J R <jayare...@hotmail.com> wrote: >Sent from my iPhone > >> On Jan 6, 2017, at 09:37, Bill Woodger <bill.wood...@gmail.com> wrote: >> >> "with the exception of the last block or track." > >Are we not talking about the "last (used) track" in the case of MOD? > >---------------------------------------------------------------------- >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
