Advanced Topics › Tape Device Allocation and Allocation Recovery Considerations › z/OS Device Allocation

z/OS Device Allocation

Allocation is the process in which z/OS evaluates the device requirements of a job and selects a device that best meets those requirements. In normal allocation processing, z/OS selects only online devices that are not allocated currently and that are suitable for that job.

The z/OS allocation mechanism creates a list of devices that the requesting job may be able to use. This list, known as the eligible device list (EDL), contains the UCB address of every suitable device. An EDL is created for each DD.

The EDL is made up of one or more device groups. For tape allocations, each device group contains a single device. z/OS chooses a device for allocation from the device groups in the EDL.

z/OS serializes the allocation process by allowing only one job to control a device group at a time. This prevents two jobs on the same system from concurrently choosing the same device.

As each DD for a job goes through allocation, z/OS requests the group locks for all the device groups in the EDL for that DD. The job must get control of all the locks for all the groups in the EDL before z/OS allows it to proceed in allocation. If another job already controls that device group, then z/OS makes the requesting job wait for the device group to be released.

Once all the device group locks are obtained for a DD, if a job cannot allocate a suitable online device, then the job enters the z/OS allocation recovery process.

The following steps give an overview of the z/OS allocation process:

1. z/OS allocation assembles the EDL for a DD for the job, based on the device groups z/OS creates during the system generation (SYSGEN) process.
2. z/OS or vendor products may mark devices in the EDL ineligible for allocation for a variety of reasons. For example, robotic software may mark devices ineligible based on whether the device is inside or outside the robot.
3. z/OS requests the device group locks for all device groups in the EDL. When all the locks are obtained, the job is allowed to proceed through allocation.
4. z/OS examines the EDL and eliminates ineligible, allocated, inaccessible, and offline devices from possible allocation. If there are no devices that can be allocated, then the job enters allocation recovery. If there is one device, then the job allocates it. If there are multiple devices, then the job goes to Step 5.
5. z/OS or vendor products may preference devices in the EDL over other devices for a variety of reasons. For example, z/OS may preference a device based on its ACL status.
6. z/OS chooses a device for allocation from the remaining devices based on the preferencing values set for the devices by z/OS, vendor products, or both.