Network Integrity LANtegrity 4.0 for NetWare
 LANtegrity for NetWare is a complete package for NetWare server mirroring. But we were surprised to find that rather than being a full real-time mirroring solution, like StandbyServer or Double-Take, LANtegrity is a hybrid, performing round-robin scanning for changes on a default schedule and real-time mirroring on a separate schedule. Configuring this approach can be somewhat confusing, and we initially missed it.
The real-time mirroring functions in LANtegrity are performed similar to those in Double-Take--via system calls in the NetWare file system. Products like StandbyServer use NetWare's mirroring functions,
which run at a lower hardware driver level. In our tests, LANtegrity, Double-Take and StandbyServer all performed comparably during a mirror of our SYS:SYSTEM directories. The round-robin-only design of LANshadow, in contrast, turned out to be a hit-or-miss situation. Depending on where the LANshadow server was in its schedule when the copy was made and the protected server crashed, it may have mirrored some or none of the files.
One feature we liked about LANtegrity was that it takes many-to-one server mirroring an extra step. With the addition of a tape changer, Network Integrity's product also can serve as a comprehensive backup or archival mechanism. Backup, including time-slice recovery from failures, has been tightly integrated into the system.
As expected, failover to the mirror server is handled smoothly by LANtegrity. Operations after the failover, however, were more sluggish compared to the other systems tested. This was because of the interaction with the tape unit, which is used to retriev
e some of the older files not kept in near-line cache, as well as the continued mirroring of the other protected servers. Clearly, heavy use with multiple servers could exacerbate this condition.
Reverting to the original server in LANtegrity requires a specific step to resynchronize the mirrored copy back to the protected server. But this step is actually much simpler than those required by LANShadow and Double-Take. They require that you reverse the mirroring process in order to resynchronize the original server's data. All of the systems we tested require rebooting the protected server. Only Network Integrity's LANtegrity, however, manages the entire reboot process automatically.
NSI Software Double-Take 2.6 for NetWare
The latest release of NSI's Double-Take proved to be a functional and straightforward product. Overall, we were very pleased with the speed and reliability of the product's real-time mirroring. However, three areas hold Double-Take back from scorin
g higher: its administrative interface; its lack of automation in failover; and its lack of automation in reverting to the original server. If your primary concern is preserving critical data, though, then once configured, Double-Take will serve you well.
The real-time mirroring of this product is accomplished via software calls that pick up the disk requests from the file system before they reach the disk drivers on the protected server. This mechanism helps to ensure that data changes sent to the mirror server are processed as soon as possible. Additionally, this mechanism maintains the integrity of the mirror server. Our problems with the administrative interface of Double-Take surfaced almost as soon as NSI's product was out of the box.
Specifically, we had a very difficult time selecting whole or even partial directory trees for mirroring--an essential task. Its checkboxes for mirroring subtrees registered only sporadically. And we encountered additional troubles with the interface when any of the
protected servers were brought down to simulate a failure. This act would lock up Double-Take's administrative interface and the entire Windows95 workstation on which it was running.
On the other hand, we found ourselves intrigued by a new facility, called fast mirroring, that is used to reconnect a protected server. In Double-Take, the file system of the protected server is scanned only for changes that occurred since the last time it was connected.
However, we noticed that a file opened on the protected server with DOS' EDIT program during a fast mirror, when finally saved, would be a corrupted copy on the mirror server. Since we were unable to find this problem with other file-lock situations or with the other products we tested, we turned to the company's technical support staff for help.
They were very responsive in getting to the root of the file system problem, and say a fix will be made available. As expected, subsequent changes to the same file forced a clean copy of the file to be sent to
the mirror.
Double-Take's core functionality is sound. We would like to see a lot more effort put into the code for resynchronizing the data once the protected server is back online. A supplied DOS program offers a decent recursive copy routine, but we found it easier to perform the data restore on the Windows95 Explorer interface.
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