Setting Up a Disaster-Recovery Location

I'm setting up a business continuity/disaster recovery location and am looking for options to replicate LUNS on my Storage Area Network (SAN) between our primary and disaster recovery sites. Our disaster recovery site is 150 miles from our primary office. How do I estimate how much bandwidth will be required to move 5GB per hour?

You need to consider many things when putting together requirements for a disaster recovery site. Because I have limited space, I'll respond in general concepts, but plenty of books on this subject are available.

First, you must determine which services and data you need to replicate to the remote site. Next, consider what supporting information the services and data might require and determine whether the data to be replicated is order-specific (e.g., don't replicate B until A has finished replicating). You also must consider tolerances for data synchronization. Can the remote site be in sync within 10 minutes? 20 minutes? An hour? This information helps you determine bandwidth requirements. After you've made these determinations and considered the environmental and equipment concerns, you can move on to the "how" of replication.

You have many options for volume-replication solutions. At the software level, you have products such as VERITAS Software's VERITAS Storage Foundation for Windows with the VERITAS Volume Replicator option enabled. Depending on your needs, you might also want to look at something like VERITAS Storage Replicator. Both tools are easy to install and relatively inexpensive to purchase and maintain.

Although Volume Replicator has quite a bit more functionality than Storage Replicator, neither product gives you the flexibility you gain from a proprietary SAN replication solution (check with your vendor) or some of the more open LUN replication solutions on the market. MaXXan (http://www.maxxan.com) provides one such product. MaXXan licensed technology from FalconStor Software (http://www.falconstor.com) and incorporated it into its SAN MXV line. Because MaXXan's solution is inline, it has direct access to data as it's written to each LUN, and FalconStor's software allows for replication and mirroring to a remote MaXXan array. This replication requires a MaXXan switch at both locations, although the company does have an agent that you can use in lieu of a switch at one of those locations. I prefer the hardware approach—it's easier to maintain and doesn't tax the host.

As for bandwidth requirements, you can probably replicate 5GB per hour over a 10Mbps connection. Depending on your area, many telecom carriers offer a higher-performance metropolitan area network (MAN) solution, but this solution is often limited to sites located within 180 miles of each other.

The most common solution of this type is Ethernet over Synchronous Optical Network (SONET), which essentially gives you a Layer 2 Ethernet handoff at each location. The advantages of this type of solution over a time-division multiplexing (TDM) solution, in which multiple low-speed signals are time-sliced into a high-speed transmission, are obvious—lower latency and no need for a dedicated router. You can also look into Multiprotocol Label Switching (MPLS) or VPN options in your area.

—Bob Chronister

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