On Monday, 05/15/2006 at 08:51 AST, Jim Bohnsack <[EMAIL PROTECTED]> wrote: > I have a question regarding "ownership" of a tape address when more than > one system has access to the control unit. It is my understanding that > since the time of the 3480 control unit, a switch or electronic latch is > set in the CU when an address is ATTACHed (VM) or ALLOCated (MVS) and that > the latch prevents another system from attaching or allocating that same > address. When the address is freed by being detached from the id on VM or > unallocated on MVS, the latch is immediately reset indicating that the > address is available. Am I correct in the above.
It depends. The latch you are talking about is called "assignment". You can attach a drive to a guest WITH ASSIGN (the default) or WITH NOASSIGN. If the guest handles assignments itself (e.g. a guest OS), then NOASSIGN is useful. CMS does not natively perform assignment, so you typically use WITH ASSIGN for a CMS user. When MVS assigns the drive to itself depends on the configuration and the subsystem. You can assign at vary online time, or you can delay to allocate time, or you can let the job do it itself. JES2 and JES3 capabilities differ. Many customers use CA's MIM, which handles the assignment itself (or doesn't use ASSIGN at all, depending on the config). There are some other latching mechanisms available as well, but they don't work with z/VM. > We occasionally see situations between or VM lpars and MVS lpars where an > address has been detached from VM but is not available on MVS. It is DFSMS > on MVS that is trying to allocate the address. If a CP Q TA is issued on > VM, and the reply is "An active tape was not found" is there any way that > the VM lpar could still own an address? QUERY TAPE ACTIVE shows you tape drives that are ATTACHed or in use by CP (e.g. SPXTAPE). If not active, then QUERY TAPE FREE shows those that are available, though a FREE tape may be assigned to another system at the moment. Now, it is possible if you ATTACH NOASSIGN, and you then DETACH the drive while the guest has the drive assigned to the VM system, CP does NOT release the assignment. The VM system still owns the drive but it will appear as FREE. You will have to VARY OFF said drive to release the assignment. Alan Altmark z/VM Development IBM Endicott
