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Real reserve/release is a DASD hardware feature that serializes access among real processors by dedicating a device to one processor at a time. Use real reserve/release processing if two or more real processors are sharing a device and you are running CA MIM on both of these processors.
Real reserve/release dedicates a device by letting only one path group control the device at a time. Because each real processor uses a unique path to access a device, real reserve/release can prevent integrity exposures among real processors.
On z/OS systems, a RESERVE request causes z/OS to issue a reserve channel command word (CCW) for a device. When z/OS runs under z/VM, the z/VM CP component sends the reserve CCW to the device on behalf of the z/OS guest.
If the device is not dedicated to a processor already, then the DASD hardware executes the reserve CCW, which dedicates the device to the channel from which the reserve CCW was issued. The device rejects other reserve CCWs until the controlling z/OS guest issues a release CCW to release the device.
z/VM may remove reserve or release CCWs from the channel program of a guest under certain circumstances. If reserve or release CCWs are removed, then an integrity exposure can occur when several real processors are sharing a device.
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