DRS, once enabled, will have a holistic view of the cluster's compute resources, and so it will have the ability to migrate VMs as per their changing runtime compute workload requirements. The cluster's compute resources are aggregated under a root resource pool. It becomes the parent resource pool for administrator-created DRS resource pools.

DRS uses vMotion to migrate VMs between the hosts in order to balance the workload on the hosts.

Balancing the compute resources will ensure that the VMs in the cluster get the resources when they need it, thereby increasing service levels. 

vCenter runs a DRS thread per cluster. Its job is to load balance the DRS cluster for better utilization of the cluster resources. It does so by migrating or generating migration recommendations for VMs whenever they're needed. It also provides initial placement for the VMs. Migration recommendations will be generated by vSphere DRS, but only when it identifies a resource imbalance in the cluster.

A resource imbalance is determined on a per-ESXi-host basis. It does so by considering the resource reservations for all of the VMs on the ESXi host, comparing these against the total capacity of the host, and then checking whether the host can or cannot meet the cumulative resource reservations of the VMs. The result will become a deviation metric, which is then compared against the migration threshold that's set on the cluster. DRS does this imbalance check on every ESXi host in the DRS cluster every 300 seconds (5 minutes). When DRS detects a cluster imbalance, it will check the migration threshold value that's set on the cluster. If the deviation that's calculated is more than the migration threshold, then DRS will generate a migration recommendation. DRS generates migration recommendations by simulating the migration of each VM on the ESXi host in order to recalculate cluster imbalance metrics. It will then choose to generate a migration recommendation for a VM whose migration would best serve in reducing the resource crunch on the ESXi host.

The behavior of vSphere DRS can be tuned by configuring an ideal automation level and a migration threshold. We will delve deeper into these in the following sections.