On Wednesday, 03/01/2006 at 12:00 CST, "Jeff Gribbin, EDS" <[EMAIL PROTECTED]> wrote: > They're fixing the VTOC to match the true size of the disk.
OK, everyone. Call off the dogs. DSF is bending over backwards to work out a solution. The PTF was actually in response to a customer who didn't like seeing the informational message that the number of cylinders reported by the device and the number recorded in the VTOC don't match. Well, *someone* is going to get the message. Further, they're trying to get rid of the "alternate cylinder" information in the VTOC since today's devices don't have alternates and never will. (At the direction of the disk drive folks.) Apparently a lot of cylinder 0's have been copied and re-copied (e.g. via DDR or flashcopy) over the years from devices that *did* have alternates to those that don't. [Don't ask my why its important to clean up the VTOC in this way...I don't know.] While CPVOL LIST is a read-type operation, having the disk linked R/W gives implicit permission for it to change the volume. It isn't unreasonable, I think, for DSF to take the opportunity to fix the VTOC w.r.t. alternate cylinders if the count is non-zero. Now, changing the cylinder count in the VTOC *may* be dangerous in that varying the volume online to MVS will (a) cause an informational message to be issued indicating a size mismatch, and (b) cause MVS to assume the remaining cylinders are available for allocation. DSF agrees with us that (b) would be a Bad Thing if it occurred just because you clipped the volume. Without the MVS threat, fixing the VTOC has no visible side effect since CP doesn't use it anyway. I think FDR, for example, will use GETDEV (or whatever the macro is called) to get the volume size, not try to use the VTOC, so backups would/should still be ok. (But if anyone can actually run the experiment....) While we research, perhaps those with more MVS experience than I can enlighten me on MVS' use of the VTOC when a mismatch occurs. (I'm told MVS has a "refresh-and-rewrite-the-vtoc" command that is used in order to fixup the vtoc...the equivalent of reallocating a cp volume after you copy it to a differently-sized volume. I didn't check to see if that was in DSF or the BCP.) Alan Altmark z/VM Development IBM Endicott
