SQL Server Magazine

Defining your company's disaster recovery plan is an essential part of LAN/WAN development. The traditional approach of performing system backups offline after hours is no longer feasible because many networks now operate around the clock and because companies can't afford to lose a day's worth of data.

One solution is Double-Take 1.3 Beta for Windows NT (DT), a clustering backup solution and resource management tool that creates continuous, realtime mirroring of data, even from files in active use. Though DT for Novell NetWare has been available for some time, Network Specialists, Inc. (NSI) released DT for NT in April. DT's purpose is to protect business operations from system failures and maintain good network availability.

DT works at the network operating system (NOS) level, not the physical driver level. Instead of copying the physical hard disk as standard mirroring does, DT replicates all OS transactions that modify the contents of the server's file system. The NOS receives and validates disk writes, and then sends the write to the primary disk and to the target system for synchronized replication. Other key features include a fault-tolerant standby server, which lets users log on directly to the backup server if the system fails. According to NSI, one of DT's strengths is WAN support of off-site replication of server data for disaster recovery.

DT supports only Intel platforms and the following configurations: one file server to one backup server, many file servers to one backup server, and one file server to many backup file servers. In the many-to-one configuration, NSI cautions against using more than 50 file servers in one backup. Available storage capacity limits the size of a backup.

Configuring for Backup
DT defines a simple backup system as three network-attached platforms: a file server, a backup file server, and a workstation. DT has four parts: DTSource, the machine that contains the original data; DTTarget, the machine that contains the replicated copy of the data; DTClient, the service that configures and monitors both machines; and the Automatic Failover Utility, which lets the target assume the role of the source. The source and target machines can be either NT servers or NT workstations. DTClient can reside on DTSource or DTTarget, or on any Windows 95 or NT system that has access to the source or the client over the network. The Automatic Failover Utility runs on the target machine.

The beta DT software I tested came on three diskettes. Installation was not painless: I couldn't install the product on any NT workstation or server within the Windows NT Magazine Lab domain. The Lab brain trust could surmise only that some previously loaded management or monitoring software conflicted with DT. Dozens of types of software can be running on the Lab's network at one time, so determining the specific culprit is nearly impossible.

For expediency, I created a new NT domain from four completely clean systems that were isolated from the Lab domain. I installed DTSource and DTClient on the Primary Domain Controller (PDC) and DTTarget on the Backup Domain Controller (BDC). I was never able to install any component on a workstation. After I installed DTClient, I started it on the PDC by selecting the DT-Client icon created during installation. As Screen 1 shows, you start DTSource and DTTarget from the Services applet in Control Panel. Both DTSource and DTTarget must be running continuously for the mirroring and failover features to work.

When the three applications are running, DTClient displays the PDC as a source machine and the BDC as a target machine, as Screen 2 shows. You simply double-click the source computer to get the Replication Set Explorer feature, shown in Screen 3. From here, you select which documents, folders, or directories you want to replicate on the target machine. Then you use the Connection Manager, shown in Screen 4, to link your source and the target by selecting the available target or targets and clicking Connect. The documentation didn't explain mirroring options well, but after several phone calls to NSI, I learned that you select the computer in the Connected box before you click the traffic light.

Because I worked with the beta version of DT, I had to use a photocopy of the page proofs of the user's manual. The documentation includes example configurations, installation instructions, and operations, but the manual falls short in other areas. In the documentation provided, some references to configuration software and operation are for Novell NetWare, not NT. Installation directions are easy to follow but fail to mention key steps, such as the need to enter an authentication number before you can install the software, and how to begin mirroring. The software did not include Help files, but NSI will add them in a later release. Although NSI said that users can download Help files from its Web site, neither the Web files nor technical support via email was available during my evaluation. NSI has since restored these services.

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