Hardware replication vs. software replication: The differences

What is hardware replication?

Many enterprise storage vendors integrate hardware-level replication capabilities into their devices. Such devices are linked by a high-speed network connection. When data is written to the primary storage device, the device automatically replicates the operation to a secondary device. This replication occurs as a function of the device firmware and isn't dependent on an external software application.

Some storage vendors design their hardware to perform block-level replication. The primary storage device keeps track of the blocks that are written or modified and then replicates those blocks to the secondary device. However, modern storage hardware is increasingly being designed to replicate snapshots rather than individual storage blocks.

Hardware replication pros and cons

• Matching hardware. Most hardware-based data replication platforms require the use of matching hardware. Although this requirement can cause issues with vendor lock-in, the bigger issue that an organization is likely to face is cost. Because the secondary site must typically be equipped with hardware that is identical to that of the primary site, it means that the organization's storage costs will effectively double.
• Replication frequency. Some hardware devices support synchronous mode, meaning that data is replicated between the two devices in real time. However, not every device supports synchronous replication. Likewise, the speed of the connection between devices may also limit the replication frequency. In these cases, asynchronous replication is used.
• Ease of troubleshooting. Hardware-based replication tends to be easier than software-based replication to troubleshoot when things go wrong. Because replication occurs between two hardware arrays, there are fewer pieces to the puzzle than there would be with software-based replication, and that greatly simplifies the troubleshooting process. Furthermore, it's easier to get technical support for hardware-based replication issues because the organization doesn't have to worry about vendors pointing fingers at one another.
• Hardware offloading. Another advantage to hardware-based replication is that certain processes can be offloaded from the production servers and performed at the storage hardware level instead. For example, it's relatively common to deduplicate data prior to replicating it. This reduces the volume of data that must be replicated. Deduplication and other similar functions can often be offloaded to the storage hardware, thereby reducing the workload that is placed on servers.

Hardware replication use cases

Hardware replication tends to be best suited for situations in which data needs to be replicated either between two data centers or between two identical storage devices within a single data center. Because hardware replication is hardware-dependent, it tends to be a poor choice for those who are on a budget.

Some storage devices do provide the option of replicating the array's contents to a public cloud, but capabilities tend to vary significantly from one device to the next.

What is software replication?

Software replication comes in many different forms but is based on the idea of using an operating system, a hypervisor or a mission-critical application to replicate data from one location to another without the replication process having to be directly supported at the hardware level.

Microsoft, for example, has built a replication feature into Hyper-V that enables VMs to be replicated to a secondary system. If the primary system fails, then the replica VM can be activated as a way of bringing the VM back online.

Similarly, many applications perform a type of database replication called logical replication. Logical replication keeps track of which objects have and haven't been replicated, based on the object's primary key. As objects are created or modified, the database replication engine migrates those changes to a replica database on the target system.

Many backup applications also include a replication feature. Backups are initially written to a designated target. That target is then replicated to a secondary location, either on premises or in the cloud, as a way of creating a redundant backup copy.

Software replication pros and cons

• Hardware-agnostic. Because the replication process is controlled by an operating system or by an application, it's largely independent of the underlying hardware. This means that an organization has the option of purchasing commodity hardware rather than investing in a high-end storage array.
• Location flexibility. Hardware-based replication generally requires matching physical devices, which limits its potential use cases. Software-based replication tends to be far more flexible because it's not constrained by this requirement. This means that it's often possible to replicate to the cloud or to create a primary replica and multiple secondary replicas.
• Application awareness. Another potential advantage to software replication is that it tends to be application aware. This is especially true if the replication engine is integrated into the application as it is for some databases. Application awareness helps to guarantee the integrity of the data residing within the replica copy.

Software replication use cases

Software replication tends to be a good choice for those who require application awareness and the flexibility to create multiple replicas in a variety of locations, although not every software-based replication engine supports multiple replicas.

It's also a good choice for organizations that don't want to have to purchase matching storage hardware. If a replica only exists for the sake of redundancy, for instance, it might make more sense to write the replica to commodity storage than to store the replica on a high-end array.

Hardware replication vs. software replication: Choosing between the two

Neither hardware- nor software-based replication is an ideal approach for every organization. Each approach has advantages and disadvantages that must be considered prior to making a purchasing decision. When deciding on a replication approach, organizations must consider factors such as their budget, the number of replicas required, existing hardware, the type of data to be replicated and the location of the replicas.

Of course, choosing a data replication type doesn't necessarily have to be an either/or decision. It's possible to implement a hybrid replication strategy that uses both software and hardware replication. For instance, an organization might use software replication to replicate a VM to a secondary data center, where the VM is then replicated to an additional storage array using hardware replication.