Product Reviews
Failover That’s Worth a Double-Take
NSI tool offers mirroring, replication and failover.
My company was contacted by a medical outfit whose data and paper records were destroyed by a tornado. Smarting from this experience, they wanted a remote backup solution that offered secure off-site backup, disaster recovery, failover and 99 percent uptime. One of my technicians had been researching NSI Software’s Double-Take and suggested we put it to the test.
We typically replicate client situations in a lab, and run a battery
of scenarios on the solutions we recommend before rollout. We were impressed
with Double-Take, which offers a suite of capabilities including mirroring,
replication and failover.
Like many backup solutions, Double-Take uses a source-target configuration.
Depending on specific needs, the source and target hardware and software
don’t have to be identical. However, when it comes to failover, the hardware
and software should really be the same on both machines. To keep in sync,
source and target machines maintain a connection which is monitored by
tools included in the software.
 |
| Figure 1. The Double-Take connection manager
console can determine what servers communicate, how often they’ll
transmit data, and what sorts of services they’ll run. |
In a typical scenario where you’re protecting one source machine, you would first mirror the source to the target and then set up real-time replication. Double-Take works at the byte level, and only replicates changes to the files. This allows a real-time, up-to-the-minute backup of the server. For failover, you can configure the target machine to assume the IP address and host name of the failed source machine. You can configure the target to poll the source machine and automatically failover to the target. Once the original source machine is repaired, the data can be restored back from the target. While relatively simple, the process does require user intervention.
In our customer’s situation, we were using a dedicated T1 bridged to a remote location, allowing the target to be located approximately eight miles from the source and still on the same subnet. During our lab and onsite testing, Double-Take performed above expectations. After deployment at the customer site, several clients were logged in to the source server. After disconnecting the power to the source server, there was approximately a 40-second period before the failover was recognized by all clients. We continued to work from the target machine, which assumed the identity of the original source, for more than an hour. Restoration procedures were implemented; however, this did require taking the servers off the network for 20 minutes.
Double-Take supports many configurations and is convenient for centralized backup. You can have many servers sending replication data to a single Double-Take target. Backups for multiple servers can be performed from one target machine while saving the resources on the source machines. You can also establish a chained configuration in which some servers act as both source and target. It’s also possible to configure a single machine as both source and target. When using different servers, you can configure customized scripts to run on the target machine before and after failover. This might be necessary to start any applications and services that might need to be running if a failover is experienced.
Double-Take can be used across VPN connections. However, anytime Double-Take’s deployed away from your local network, bandwidth becomes a consideration. Double-Take only replicates the changes to a file, which saves significant bandwidth. Also, the replication can be queued locally on the source and scheduled to be sent to the target at a convenient time.
The bandwidth that Double-Take’s allowed to use can also be set. It has a null target built in that imitates a normal connection without actually passing data across the network. This can generate statistics that approximate the bandwidth needed when an active connection is established.
Although we’re thoroughly pleased with this product, no off-the-shelf package is ever perfect. On disadvantage is that Double-Take won’t work across a NAT router. And while the Double-Take Management Console GUI is fairly simple to use once you’ve adapted to the lexicon, it could be more intuitive. For more direct control, there’s a command-line utility.
You can download a demo from the NSI Web site, and the product documentation
is fairly straightforward and accurate.
About the Author
Ben Brady, MCSE, CCNP, is general manager of The MultiPro Network, a Tennessee-based company that offers network services, Web development and training.