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Virtual reserve/release is a z/VM feature that serializes access among guests running under the same z/VM operating system. Use virtual reserve/release processing if you are running more than one guest under the same z/VM system and you share devices among these guests.
You must invoke virtual reserve/release in this situation because guests under the same z/VM operating system share the same real channel to a device. Real reserve/release cannot ensure integrity among these guests, because it has no effect on guests that share a real channel to a device.
Virtual reserve/release dedicates a specified minidisk to a single guest at a time. You can use virtual reserve/release to dedicate part of a device to a guest (if you defined several minidisks on that device) or to dedicate an entire device to a guest (if you defined that device as a full-pack minidisk). The implications of dedicating only part of a device are discussed in detail in the next section.
During virtual reserve/release processing, CP intercepts a reserve CCW, marks the appropriate minidisk as reserved, and then sends the reserve CCW to the device. You may need to tell z/VM systems to send reserve CCWs properly, as described in Telling z/VM to Send Reserve CCWs to Devices in this chapter. The device then performs real reserve/release processing in response to the reserve CCW, which prevents other real processors from accessing the device while it is busy. CP prevents other guests running under the same z/VM operating system from accessing that minidisk until the controlling guest issues a release CCW. CP also sends the release CCW to the device so that other processors can access the device again.
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