You could call it a documentation error of omission and fix it that way :) It would be (a) easier, and (b) backward compatible.
Regards, Richard Schuh > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:[EMAIL PROTECTED] On Behalf Of Alan Altmark > Sent: Friday, November 14, 2008 3:13 PM > To: [email protected] > Subject: Re: FW: Risk of Adding a Paging Volume > > On Friday, 11/14/2008 at 01:07 EST, "Schuh, Richard" <[EMAIL PROTECTED]> > wrote: > > Actually, it is record 3 that contains the volume label. It and > > records 1, and 2 are 80 byte records. > > Technically, the VOL1 label standard permits the label to > exceed 80 characters, up to the record length, but CPVOLs > only use 80 bytes. > > > 1 and 2 are the IPL1 and IPL2 records if the volume can be IPLed. > > Records 4 and 5 are the VTOC that is written to prevent > other systems > > from corrupting the volume. > > [Notation below is "cylinder/head(track)/record".] > > 0/0/4 contains the allocation map. It is able to go there > because the label (0/0/3) points to the record (on 0/0) that > contains the VTOC. CPVOLs (i.e. CPFMTXA) place the VTOC on 0/0/5. > > The VTOC contains two extents: > 1. The extent of the VTOC itself. This is where it gets > clever. The label says it starts at 0/0/5, but VTOCs are > measured in *tracks*, not *records*. For CPVOLs, the VTOC > consumes all of cyl 0, track 0. This protects all the > records on track 0 from the depredations of z/OS (assuming it > was so inclined). > > 2. The list of available extents. Being oh so clever again, > the list is empty. > > If anyone wants to read more about VTOCs, and DSCBs, go look > at the z/OS DFSMSdfp Advanced Services book, chapter 1. > > > The system has, for a long time, protected the first records of > > cylinder 0, maybe all of 0/0/0, and used the rest of the > cylinder for > > paging or spooling if cylinder 0 is so allocated. > > Note that this protection does not extend to T-disk. If you > allocate cyl 0 as TDSK, CP will happily (for the moment) hand > real cyl 0 to a guest. So the rule is "don't do that". We > plan to fix it in a future release. > > Alan Altmark > z/VM Development > IBM Endicott >
