Eligible Devices Table (EDT)

During a normal system operation, there will only be one EDT. However, during a dynamic configuration change, the system will use more than one EDT to handle the configuration change. During a dynamic configuration change, the following EDTs may be bult:

Original EDT – this is the EDT that was in use before the configuration change began.
Intermediate EDT – this EDT is only build when devices are being removed from the configuration and it contains the devices from the original EDT that are being removed. If devices are being added to the configuration, those devices are not part of the intermediate EDT.
Final EDT – this EDT is the EDT that will be in use once the configuration change is complete and contains all of the devices being added and does not contain any devices that were removed.

Although there may be as many as three EDTs built during a dynamic configuration change, there will be only one or two EDTs present at a time. When devices are not being deleted from the configuration, the system will only build the final EDT and transition from the original EDT directly to the final EDT. Once that is complete, the original EDT will be deleted. When devices are being deleted from the configuration, the system will first build an intermediate EDT and transition from the original EDT to the intermediate EDT. Once that is complete, the system will then delete the original EDT and build the final EDT. Once that is complete, the intermediate EDT is deleted.

The system also uses the following two terms to describe an EDT:

Primary EDT — an EDT that processes all new allocation requests.
Secondary EDT — an EDT that processes all allocation requests that have not yet completed and are not used for new requests.

During a dynamic configuration change, when the system is transitioning from one EDT to another, the secondary EDT is the EDT that the system is transitioning from and the primary EDT is the EDT that the system is transitioning to. For example, when the system is deleting devices from the configuration, the system first transitions from the original EDT to the intermediate EDT and then from the intermediate EDT to the final EDT. In this case, when the system is transitioning from the original EDT to the intermediate EDT, the original EDT is the secondary EDT and the intermediate EDT is the primary EDT. Once that transition is complete, the system then transitions from the intermediate EDT to the final EDT. At this point, the intermediate EDT becomes the secondary EDT and the final EDT becomes the primary EDT.

An EDT transition cannot complete until all allocation requests that are using the secondary EDT complete.

In summary, the original, intermediate, and final EDT terms describe the physical EDTs that are created by the system. Over the course of a dynamic configuration change, these terms do not change. The primary and secondary EDT terms describe the logical EDT that is being transitioned from or transitioned to and also describes which EDT is being used to allocate new requests. Over the course of a dynamic configuration change, the primary and secondary EDT may describe different physical EDTs at different points in time.

The secondary EDT receives no new allocation requests. The system removes it when it finishes processing the allocation requests that use the secondary EDT. An EDT transition cannot complete until all allocation requests that are using secondary EDT complete.

When the system has only one EDT, it is known as the primary EDT. The EDT created at IPL, for example, is initially described as the primary EDT.

As you diagnose problems with the allocation component, be aware that dynamic configuration adds additional EDTs to the system.