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VSphere Replication is a feature of vSphere 5.0 and Site Recovery Manager (SRM) 5.0 that automates the failover of virtual servers to a recovery site. vSphere Replication enables the replication of virtual machines (VMs) at the virtual layer instead of at the storage layer, as was required with earlier versions of vSphere and SRM.
With replication built into Site Recovery Manager, customers no longer need to rely on third-party storage applications and arrays for replication. Its hypervisor -based replication also removes the issue of compatibility between arrays at the primary and secondary sites. Unlike array-based replication, however, vSphere Replication does not support automatic failback. Failback, which is the process of re-synchronizing data back to the primary location, must be done manually.
vSphere Replication uses vSphere’s Changed Block Tracking (CBT) feature, copying only changed blocks to the recovery site. This lowers bandwidth utilization and makes it easier to achieve lower recovery point objectives (RPOs) than doing a full-system copy of VMs. vSphere Replication supports RPOs from 15 minutes to 24 hours.
Although it is built into vSphere, vSphere Replication requires a separate license and consists of three components -- vSphere Replication Management Server (VRMS), vSphere Replication Server (VRS) and vSphere Replication Agent (VRA). VRMS and VRS are virtual appliances. vSphere Replication Management Server allows the administrator to centrally manage replication across hosts, while vSphere Replication Server performs the actual data replication.