The previous sections describe how to attach to multiple guest systems when the guests are all full-time production systems. You may also want to support guests that only operate occasionally or only need the use of tape drives part-time, such as during a backup procedure. If CA MIA for z/VM is managing the devices on the z/VM host system, then the SYSTEM option on the MI ATTACH command can be very useful in this situation.
If you attach using the SYSTEM option, then two rules must be observed:
If either rule is violated, then dual allocation of the drive can occur. The second requirement is the same as the requirement placed on drives that are permanently attached to the guest systems. It is mentioned here because the manual use of the ATTACH command is more prone to error than other attach methods, such as a DEDICATE statement in the z/VM directory.
When you use the SYSTEM option, the guest system participates as a peer with all other systems that share the same drive. If the guest allocates the drive, then it blocks allocation on all other systems. This implies that before the guest can allocate, the drive must be free on all other systems.
The key feature of the SYSTEM option is that it prevents the drive from looking allocated due to the ATTACH command at the z/VM host level. If the SYSTEM option were not used in this situation, then neither the guest system nor any other system would be able to use the drive, because the attach to the guest would count as an allocation of the device.
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