A DRS cluster is a collection of ESXi hosts and associated virtual machines with shared resources and a shared management interface. Before you can obtain the benefits of cluster-level resource management you must create a DRS cluster.

When you add a host to a DRS cluster, the host’s resources become part of the cluster’s resources. In addition to this aggregation of resources, with a DRS cluster you can support cluster-wide resource pools and enforce cluster-level resource allocation policies. The following cluster-level resource management capabilities are also available.

Load Balancing

The distribution and usage of CPU and memory resources for all hosts and virtual machines in the cluster are continuously monitored. DRS compares these metrics to an ideal resource utilization given the attributes of the cluster’s resource pools and virtual machines, the current demand, and the imbalance target. It then performs (or recommends) virtual machine migrations accordingly. See Virtual Machine Migration. When you first power on a virtual machine in the cluster, DRS attempts to maintain proper load balancing by either placing the virtual machine on an appropriate host or making a recommendation. See Admission Control and Initial Placement.

Power management

When the vSphere Distributed Power Management (DPM) feature is enabled, DRS compares cluster- and host-level capacity to the demands of the cluster’s virtual machines, including recent historical demand. It places (or recommends placing) hosts in standby power mode if sufficient excess capacity is found or powering on hosts if capacity is needed. Depending on the resulting host power state recommendations, virtual machines might need to be migrated to and from the hosts as well. See Managing Power Resources.

Affinity Rules

You can control the placement of virtual machines on hosts within a cluster, by assigning affinity rules. See Using DRS Affinity Rules.

Depending on whether or not Enhanced vMotion Compatibility (EVC) is enabled, DRS behaves differently when you use vSphere Fault Tolerance (vSphere FT) virtual machines in your cluster.

DRS Behavior with vSphere FT Virtual Machines and EVC

EVC

DRS (Load Balancing)

DRS (Initial Placement)

Enabled

Enabled (Primary and Secondary VMs)

Enabled (Primary and Secondary VMs)

Disabled

Disabled (Primary and Secondary VMs)

Disabled (Primary VMs)

Fully Automated (Secondary VMs)