One of the best mechanisms for protecting Hyper-V VMs in smaller organizations has historically been the replication feature. First introduced in Windows Server 2012, replication creates a synchronized VM copy on a secondary Hyper-V server. That way, if something were to happen to the primary VM, the secondary copy can be brought online and used.
Disadvantages of Hyper-V replication
Hyper-V replication is a very useful feature, especially for smaller shops. Even so, the replication feature isn't perfect. The main disadvantage to using VM replication is that the technology doesn't scale very well. Administrators must enable replication on a per-VM and per-virtual hard disk basis. The Hyper-V Manager doesn't provide a way of bulk enabling replication.
A second disadvantage to Hyper-V replication is that it doesn't provide for automatic failover. If the primary copy of a VM were to fail, then an administrator would have to manually activate the VM copy. When doing so, there's a potential for at least some data loss to occur. Hyper-V uses asynchronous replication, which means that replication doesn't happen in real time. In the event that an unplanned failover is required, any write operations that have occurred since the last replication cycle will be lost.
Introducing Storage Replica
Hyper-V replication still exists in Windows Server 2016, but Microsoft has also introduced a new type of replication, which it refers to as the Storage Replica feature or simply storage replication. Storage Replica isn't a Hyper-V specific feature, but rather a Windows Server feature. Although Storage Replica can be used in conjunction with Hyper-V, that isn't its only use case. The feature is also commonly used for file server replication.
Windows Server 2016 Storage Replica has two main advantages over Hyper-V replication. First, Storage Replica works at the storage volume level, rather than at the virtual hard disk level. This means that administrators can replicate an entire storage volume, rather than having to enable replication for each VM.
A second advantage to using Storage Replica is that it can be used either synchronously or asynchronously. Hence, it's possible to perform block level replication in near real time.
Being that Windows Server Storage Replica is still relatively new, the feature does have its limitations. One such limitation is that the feature currently only allows for one-to-one replication. In contrast, Microsoft introduced an extended replication feature for Hyper-V in Windows Server 2012 R2. This allows an organization to create two VM replicas. An organization might, for instance, create an on-premises replica of a VM, and store a secondary replica in an off-site location. Although the Windows Storage Replica feature doesn't have this capability yet, there have been numerous hints that Microsoft may allow for one-to-many replication in a future Windows Server version.
Storage Replica vs. Hyper-V replication
The Storage Replica feature found in Windows Server 2016 is slightly more flexible than the Hyper-V replication feature in terms of the supported replication topologies. Although the Hyper-V replication feature is widely regarded as a mechanism for replicating VMs from one stand-alone Hyper-V server to another, Hyper-V does support other replication topologies. The Hyper-V replica feature can be used to enable cluster to cluster replication, stand-alone Hyper-V server to cluster replication or cluster to stand-alone Hyper-V server replication.
Like Hyper-V, the Windows Storage Replica feature supports server-to-server replication and cluster-to-cluster replication. The difference, of course, is that replication is occurring at the storage volume level rather than at the VM level. In addition, the Storage Replica feature also supports a stretch cluster configuration.
Stretch clusters are failover clusters in which nodes are geographically separated from one another. The Storage Replica feature can benefit stretch clusters by placing a synchronized copy of the cluster shared volume in both of the data centers that are serviced by the cluster.
Like any new technology, the Windows Storage Replica feature has its advantages and disadvantages. Hyper-V admins can use the feature to perform synchronous replication at the storage volume level, but unlike Hyper-V replication, Storage Replica doesn't allow for the creation of extended replicas. If native Hyper-V replication is fully meeting the needs of an organization, then there is probably no reason to consider an alternate replication mechanism. If, however, more scalable replication is needed, then the Windows Storage Replica feature may be a good choice.