Distributed Power Management (DPM) and Distributed Resource Scheduler (DRS) are not mandatory, but Site Recovery Manager supports both services and enabling them provides certain benefits when you use Site Recovery Manager.
DPM is a VMware feature that manages power consumption by ESX hosts. DRS is a VMware facility that manages the assignment of virtual machines to ESX hosts.
Site Recovery Manager temporarily disables DPM for the clusters on the recovery site and ensures that all hosts in the cluster are powered on when recovery or test recovery starts. This allows for sufficient host capacity while recovering virtual machines. After the recovery or test is finished, Site Recovery Manager restores the DPM settings on the cluster on the recovery site to their original values.
For planned migration and reprotect operations, Site Recovery Manager also disables DPM on the affected clusters on the protected site and ensures that all of the hosts in the cluster are powered on. This allows Site Recovery Manager to complete host level operations, for example unmounting datastores or cleaning up storage after a reprotect operation. After the planned migration or reprotect operation has finished, Site Recovery Manager restores the DPM settings on the cluster on the protected site to their original values.
The hosts in the cluster are left in the running state so that DPM can power them down as needed. Site Recovery Manager registers virtual machines across the available ESX hosts in a round-robin order, to distribute the potential load as evenly as possible. Site Recovery Manager always uses DRS placement to balance the load intelligently across hosts before it powers on recovered virtual machines on the recovery site, even if DRS is disabled on the cluster.
If DRS is enabled and in fully automatic mode, DRS might move other virtual machines to further balance the load across the cluster while Site Recovery Manager is powering on the recovered virtual machines. DRS continues to balance all virtual machines across the cluster after Site Recovery Manager has powered on the recovered virtual machines.