We just had some fun with tape MLBs.
We have a small IBM tape library connected to our production systems. We recently acquired another system without a tape drive.
Although probably not best practice, we've run a fiber from the new system to the tape library. Then we swap the fiber between the production system and the new system. Everything's been working OK, except for the warning messages that a fiber connection failed.
The fun we had:
We connected the new system and allocated the drive to the MLB. When we were finished, we did not de-allocate the drive, and just swapped the fiber back to the production system.
When we re-connected the production system, we could not allocate the drive, from any of the production LPARs. Message was 0206, drive is assigned to another system. We could see the tapes listed in the library, and removing a cartridge from the library did update the list of tapes. So the connection was not the issue.
Only when we re-connected the new system and de-allocated the drive there were we able to use the drive on the production systems.
Apparently, either the library or the drive keeps track of the system it's allocated to in order to prevent problems! Which does make sense; the production system has 4 LPARs that share the drive, and only one can allocate the drive at a time. The surprise was that it happened with a manually-swapped fiber connection!
Paul E Musselman