vSphere DRS provides load balancing of the cluster by using vSphere vMotion to migrate workloads from heavily-loaded hosts to less-utilized hosts in the cluster. It can be set up to operate in the following modes:

• Manual: Recommendations are made but not acted upon unless an administrator initiates the migration.
• Partially automated: Recommendations are made but not acted upon unless an administrator initiates the migration, but initial power-on placements are made automatically.
• Fully automated: Initial power-on placement is automatic and all recommendations are acted upon automatically by the system.

How aggressively vSphere DRS operates depends on the migration threshold value that is set. The threshold value has five different levels, ranging from conservative to aggressive. The lower the setting, the less likely a migration is to occur, unless certain criteria happen, such as the system entering maintenance mode or if there is a significant projected performance gain as a result of the migration. The highest threshold level accepts any migration request generated by the system, regardless of how big the performance gain might be. The third level provides the best compromise between load balancing and reducing the number of vSphere vMotion events.