Home > Storage > PowerStore > Data Protection > Dell PowerStore: Metro Volume > Theory of operation
You can create a Metro Volume that spans between two PowerStore clusters. The option to configure Metro Volume is available in PowerStore Manager > Volume or Volume Group overview, or in the subtab Protection > Metro Volume, and it requires a remote system configuration. When you select the Remote System and the Metro Volume creation starts, PowerStore configures a metro replication session and starts the initial sync of the volume to the remote system. When the remote system is a multi-appliance PowerStore cluster, PowerStore can place the volume or volume group automatically on the best applicable appliance on the remote PowerStore cluster. Alternatively, PowerStore Manager allows you to select a specific appliance in the Metro Volume configuration. During initial synchronization, even though the volume is not usable on a remote PowerStore for host I/O, the new Metro Volume could already be mapped to hosts. For initial synchronization, PowerStore uses the same snapshot-based asynchronous replication process as described in the document PowerStore: Replication Technologies. While the Metro Volume is not fully synchronized between the PowerStore clusters, asynchronous replication cycles are continuously running. For final synchronizations, the replication changes from an incremental-based replication to a differential synchronization.
When the Metro Volumes are fully synchronized, PowerStore changes into the active-active state and enables host I/O on the remote PowerStore cluster. The duration for initial synchronization depends on the amount of data on the source and host write activity to the volume. A single internal snapshot is created after initial synchronization. The internal snapshot is used as a common base when a Metro Volume must be resynchronized, for instance, after a pause or fracture. If a new sync is required, PowerStore uses the common base for an asynchronous replication until Metro Volume is synchronized on both PowerStore clusters and switches into the active-active state. When the status shows Operating Normally (active-active), hosts can perform concurrent I/O on both PowerStore clusters simultaneously. PowerStore controls the active path with implicit Asynchronous Logical Unit Access (ALUA). Each Metro Volume on an appliance has ALUA active-optimized paths to the volume on one node. It also has ALUA active-non-optimized paths to the same volume on the other nodes. However, a host can use multiple paths to both nodes for I/O. The node affinity of the individual volume is the determining factor in selecting the active-optimized path for an appliance.