Metro Mirror and Global Mirror for ESS support the capability to initiate failover to the secondary site in the case of a disaster or planned outage at the primary site. Once the primary site is recovered, you can initiate failback to the primary site from the secondary site, restoring your original primary-secondary volume relationships. See PPRC failover/failback for details.
Although failover/failback is supported by the ESS, it is intended for use with Global Copy only in a planned outage scenario.