In a parent/child hierarchy of resource pools and virtual machines, the single resource pool that has no parent pool is known as the root resource pool.

Configuration

A resource pool is configured with a set of CPU (in MHz) and memory (in MB) resources. These resources are specified in absolute terms with a resource reservation and a resource limit, along with a shares setting. The shares are used during resource contention, to ensure graceful degradation.

For the root resource pool, the values of the reservation and the limit are set by the system and are not configurable. The reservation and limit are set to the same value, indicating the total amount of resources the system has available to run virtual machines. This is computed as the aggregated CPU and memory resources provided by the set of current available hosts in the parent compute resource minus the overhead of the virtualization layer.

Since the resource pool configuration is absolute (in MHz or MB), the configuration can become invalid when resources are removed. This can happen if a host is removed from the cluster, if a host becomes unavailable, or if a host is placed in maintenance mode. When this happens, the system flags misconfigured resource pools and displays the reservations and limits that are in effect. Further, in a DRS enabled cluster, the tree can be misconfigured if the user bypasses VirtualCenter and powers on VMs directly on the host.

A General Discussion of Resource pool states and admission control There are three states that the resource pool tree can be in: undercommited (green), overcommited (yellow), and inconsistent (red). Depending on the state, different resource pool configuration policies are enforced. The states are described in more detail below:

• GREEN (aka undercommitted): We have a tree that is in a good state. Every node has a reservation greater than the sum of the reservations for its children. We have enough capacity at the root to satisfy all the resources reserved by the children. All operations performed on the tree, such as powering on virtual machines, creating new resource pools, or reconfiguring resource settings, will ensure that the above constraints are maintained.
• RED (aka. inconsistent): One or more nodes in the tree has children whose reservations are greater than the node is configured to support. For example, i) a resource pool with a fixed reservation has a running virtual machine with a reservation that is higher than the reservation on resource pool itself., or ii) the child reservations are greater than the limit.

  In this state, the DRS algorithm is disabled until the resource pool tree's configuration has been brought back into a consistent state. We also restrict the resources that such invalid nodes request from their parents to the configured reservation/limit, in an attempt to isolate the problem to a small subtree. For the rest of the tree, we determine whether the cluster is undercommitted or overcommitted according to the existing rules and perform admission control accordingly.

  Note that since all changes to the resource settings are validated on the VirtualCenter server, the system cannot be brought into this state by simply manipulating a cluster resource pool tree through VirtualCenter. It can only happen if a virtual machine gets powered on directly on a host that is part of a DRS cluster.

• YELLOW (aka overcommitted): In this state, the tree is consistent internally, but the root resource pool does not have the capacity at to meet the reservation of its children. We can only go from GREEN -> YELLOW if we lose resources at the root. For example, hosts becomes unavailable or is put into maintenance mode. Note that we will always have enough capacity at the root to run all currently powered on VMs. However, we may not be able to satisfy all resource pool reservations in the tree. In this state, the reservation configured for a resource pool is no longer guaranteed, but the limits are still enforced. This provides additional flexibility for bringing the tree back into a consistent state, without risking bringing the tree into a RED state. In more detail:
  • Resource Pool The root is considered to have unlimited capacity. You can reserve resources without any check except the requirement that the tree remains consistent. This means that nodes whose parents are all configured with expandable reservations and no limit will have unlimited available resources. However, if there is an ancestor with a fixed reservation or an expandable reservation with a limit somewhere, then the node will be limited by the reservation/limit of the ancestor.
  • Virtual Machine Virtual machines are limited by ancestors with a fixed reservation and the capacity at the root.

Destroying a ResourcePool

When a ResourcePool is destroyed, all the virtual machines are reassigned to its parent pool. The root resource pool cannot be destroyed, and invoking destroy on it will throw an InvalidType fault.

Any vApps in the ResourcePool will be moved to the ResourcePool's parent before the pool is destroyed.

The Resource.DeletePool privilege must be held on the pool as well as the parent of the resource pool. Also, the Resource.AssignVMToPool privilege must be held on the resource pool's parent pool and any virtual machines that are reassigned.

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