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Smaller and medium-sized businesses are less likely to be offered these sorts of deals on arrays to get them to adopt array-based replication, but there are a couple of reasons why host-based replication may not be for them.

First, there's the overhead issue, which becomes particularly acute if the servers in question are already highly utilized.

Host-based replication is also not suited to transactional databases — native database replication tools provide a much better solution.

There's also the problem that host-based replication agents run on top of operating systems, and the majority of products are targeted at Windows environments. But some vendors do offer solutions for Linux environments as well — notably Vision Solutions' Double-Take and SIOS Technology's SteelEye DataKeeper.

These problems are real, but they don't apply to the majority of SMBs. That explains why host-based replication is so widely adopted in this market, Dines says.

Hypervisor-Based Replication

There is one case where host-based replication appeals to larger enterprises as well as SMBs. That's where the server infrastructure is largely virtualized, and the replication agent sits in the hypervisor. (There's a good argument for saying that strictly speaking this is hypervisor-based replication, something distinct from host-based replication. But since the benefits — lower costs, the ability to use heterogeneous hardware and so on — are the same, we will treat it as a form of host-based replication for the purposes of this article.)

One reason is that hypervisor-based replication, unlike conventional host based replication, is highly scalable because the resource overhead for hypervisor-based replication is much lower.

And hypervisor-based replication can be carried out easily using "native" systems — VMware includes replication in Essentials Plus, Standard, Enterprise and Enterprise Plus versions of vSphere, while Microsoft includes Hyper-V Replica in all versions of Windows that include Hyper-V. And it can be managed from virtualization management suites like VMware's vCenter or Microsoft's System Center. Specialist vendor Zerto also provides hypervisor based replication for VMware VMS which can be managed through vCenter.

Since hypervisor-based replication is "VM-aware," it is possible to select the VMs that need to be replicated, while saving storage space at the secondary site by avoiding replicating the ones that don't.

By contrast, array-based replication can generally only replicate an entire volume. That means that, desirable or not, it's necessary to replicate every VM on that volume. It also means that there has to be enough capacity at the remote site to accommodate that volume, even though replicas of some of those VMs may not be needed. It's possible to get around this by creating different volumes for different groups of VMs — those that need to be replicated, and those that don't. But that makes storage management very much more complex.

"Hypervisor-based replication allows you to be much more granular in what you protect, and it also allows you to group VMs by defining protection groups," says Dine. Protection groups may contain all the VMS that make up one application, and by grouping them they can be protected and recovered together, providing complete consistency regardless of physical location on servers and storage. Hypervisor-based replication also makes recovery easier because it provides control over the order that systems are recovered, Dine adds.

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