Actually, it is record 3 that contains the volume label. It and records 1, and 2 are 80 byte records. 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. These records are, IIRC, 96 bytes with a 44 byte Key field. If the volume has actual IPL records instead of dummies, the next record is likely to be the one that is read by the IPL channel program. There may be more than one such record. The first record read it the first page of the IPL program. It contains the low memory stuff such as the IPL PSW and addresses of the First Level Interrupt Handlers for the IPL program.
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. Regards, Richard Schuh > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:[EMAIL PROTECTED] On Behalf Of Tom Duerbusch > Sent: Friday, November 14, 2008 8:58 AM > To: [email protected] > Subject: Re: FW: Risk of Adding a Paging Volume > > The first 3 pages of cylinder zero of a CP volume, have > specific, non-page, applications. > > Obviously, page 0, contains the volume label. Page 1 and 2 > have the CP Allocation map (the map that shows what each > cylinder can be used for...temp, page, spol, perm, etc). > > It use to be, I don't know if it is the case now, that the > paging subsystem, used all pages of the cylinder marked as > PAGE. If it used page 0 or 1, there went your volume header > and page allocation map. No big deal as these are stored in > memory on a running system. But once you IPL, and CP tried > to read these pages..... > > Back in the 2314 days, we sweated every cylinder. Now, we > only sweat on a pack basis. <G> > > Just lay off cylinder 0 on any volume that isn't just > attached to a guest. > > Tom Duerbusch > THD Consulting > > >>> Edward M Martin <[EMAIL PROTECTED]> 11/14/2008 9:56 AM >>> > Hello Everyone, > > Just been following this thread. Could some one > explain why Cylinder 0 should not be defined as page? > > Ed Martin > Aultman Health Foundation > 330-588-4723 > ext 40441 > > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:[EMAIL PROTECTED] On Behalf Of Mike Walter > Sent: Friday, November 14, 2008 9:50 AM > To: [email protected] > Subject: Re: Risk of Adding a Paging Volume > > No matter. No harm would be done.... CPFMTZA wouldn't execute anyway. > ;-) > > Besides, cyl 0 as PAGE would not cause a failure, it's just > not "best practices". > > Mike Walter > Hewitt Associates >
