Hyper-V replicas provide a simple and economical way to create virtual machine replicas in real time at a disaster-recovery site. Because replication is a disaster-recovery feature, administrators often ask whether Hyper-V is capable of replicating virtual machines through low-bandwidth links, such as a wide area network between data centers.
In general, Hyper-V replicas can work on a low-bandwidth link, but there are several things to consider. First copy process
One of the first factors to consider is the first replication process. During this process, Hyper-V copies the virtual hard disk to the replica server.
In a low bandwidth environment, the first replication process across the network should be avoided. First, this process may take a long time to complete, unless you enable the quality of Service (QoS) network features or similar mechanisms, otherwise this process will preempt the bandwidth of other processes.
Another reason to avoid replicating virtual machines in slow links is that virtual opportunities change quickly in some cases. Any changes to the virtual machine will not cause problems until the first replication process, but bandwidth throttling makes it almost impossible to synchronize all changes in real time after the first replication process completes.
In a network-based environment-even for high-bandwidth network environments-the first time a large virtual machine is replicated, Hyper-V may have some problems. In a way, the first replication failure is even a common scenario, even if multiple terabytes of virtual disks have been replicated, and even a lot of available bandwidth has to be rerun.
For these reasons, it is a good idea to use another way to complete the first replication process for a virtual machine. Microsoft allows you to complete the first copy of the virtual machine by exporting the replica version of the virtual machine to a removable device, or by using a replicated version of a virtual machine that already exists on the replica server (for example, you might have saved a backup version of the virtual machine on the replica server).
Some administrators find that using an existing virtual machine replica on a replica server as an initial source of replication may cause problems. In some cases, using this method can cause synchronization to fail. Therefore, the best way to do this is to export the virtual machine copy to a removable device and then import the virtual machine to the replica server to complete the first replication process.
Virtual Machine Change Frequency
Another factor to consider when making virtual machine replication in a low-bandwidth network is the frequency at which a virtual machine changes. For example, you set the virtual machine to replicate every 5 minutes, and the average 100MB of virtual opportunity changes every five minutes. If your wide area network (WAN) connections are fast enough to sync 100MB changes every five minutes, then theoretically you don't have to worry about anything. Even so, you still have to consider the bandwidth footprint problem. Because you don't want virtual machine replication traffic to consume all the available bandwidth on the network.
Even if you have a dedicated traffic line only for transmission, you still need to be careful not to use all traffic. Imagine that if the primary service is interrupted, then Hyper-V will fail the replication process. At the beginning of the next replication process, Hyper-V will have to replicate twice times the normal data. It is not necessary to replicate all the extra data traffic within a single replication cycle, but you need to ensure that there is sufficient bandwidth available to allow Hyper-V to complete these incremental replications within a reasonable amount of time.
How much bandwidth do you need?
Unfortunately, there is no single, universal solution now. Each virtual machine has a different situation. The best way to determine the bandwidth requirements of a replication process is to temporarily replicate all virtual machines to a lab server, and then measure the amount of data traffic that is emitted per replication cycle. You may need to monitor the replication process for at least one weeks to get accurate data on the bandwidth requirements of this replication cycle.
The Hyper-V replica feature still works with low bandwidth connectivity, as long as the available bandwidth can replicate the changes in the virtual machine in a timely manner. However, it is important to remember that the Hyper-V replica features are poorly extensible. For a large-scale virtual machine, its simple features do not monitor bandwidth requirements. In this case, large enterprises tend to replicate the storage area network rather than replicate the virtual machines.