Introduction to VMware DRS and VMware HA Clusters

Clusters are useful primarily with VMware DRS and VMware HA. This guide only gives a brief introduction. See the following manuals for details:

■	VMware DRS. vSphere Resource Management Guide

■	VMware HA. vSphere Availability Guide

VMware DRS

A VMware DRS cluster is a collection of ESX/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, a DRS cluster supports cluster-wide resource pools and enforces cluster-level resource allocation policies. The following cluster-level resource management capabilities are available.

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Load Balancing. The vCenter Server system monitors distribution and usage of CPU and memory resources for all hosts and virtual machines in the cluster. 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. DRS then performs (or recommends) virtual machine migrations. When you first power on a virtual machine in the cluster, DRS attempts to maintain proper load balancing either by placing the virtual machine on an appropriate host or by making a recommendation.

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Power Management. When the VMware DTM (Distributed Power Management) feature is enabled, DRS compares cluster- and host-level capacity to the demands of the cluster’s virtual machines, including recent historical demand. DTM places (or recommends placing) hosts in standby power mode if sufficient excess capacity is found. DTM powers on (or recommends 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.

■	Virtual Machine Placement. You can control the placement of virtual machines on hosts within a cluster, by assigning DRS affinity or antiaffinity rules.

See Managing DRS Clusters.

VMware HA

VMware HA supports high availability for virtual machines by pooling them and the hosts they reside on into a cluster. VMware HA monitors the hosts. In the event of host failure, VMware HA migrates virtual machines to hosts with capacity. When you add new virtual machines to a VMware HA cluster, VMware HA checks whether enough capacity to power on that virtual machine on a different host in case of host failure is available.

See Managing HA Clusters.