Although DRS performs initial placements so that load is balanced across the cluster, changes in virtual machine load and resource availability can cause the cluster to become unbalanced. To correct such imbalances, DRS generates migration recommendations.

If DRS is enabled on the cluster, load can be distributed more uniformly to reduce the degree of this imbalance. For example, the three hosts on the left side of the following figure are unbalanced. Assume that Host 1, Host 2, and Host 3 have identical capacity, and all virtual machines have the same configuration and load (which includes reservation, if set). However, because Host 1 has six virtual machines, its resources might be overused while ample resources are available on Host 2 and Host 3. DRS migrates (or recommends the migration of) virtual machines from Host 1 to Host 2 and Host 3. On the right side of the diagram, the properly load balanced configuration of the hosts that results appears.

Figure 1. Load Balancing

This figure shows how DRS rebalances a cluster.

When a cluster becomes unbalanced, DRS makes recommendations or migrates virtual machines, depending on the default automation level:

  • If the cluster or any of the virtual machines involved are manual or partially automated, vCenter Server does not take automatic actions to balance resources. Instead, the Summary page indicates that migration recommendations are available and the DRS Recommendations page displays recommendations for changes that make the most efficient use of resources across the cluster.
  • If the cluster and virtual machines involved are all fully automated, vCenter Server migrates running virtual machines between hosts as needed to ensure efficient use of cluster resources.

    Note: Even in an automatic migration setup, users can explicitly migrate individual virtual machines, but vCenter Server might move those virtual machines to other hosts to optimize cluster resources.

By default, automation level is specified for the whole cluster. You can also specify a custom automation level for individual virtual machines.