HADR Brings Resiliency to Virtualization

Stratus Technologies Avance 1.3

Stratus Technologies Avance 1.3 is a rival to Marathon's everRun VM in that it provides automatic failover. Like everRun VM, Avance runs on Citrix XenServer. Stratus sells a customized version of XenServer as part of the product, however, hardened and modified to support Avance's failover services. Unlike Marathon, Stratus supports Linux as well as Windows instances.

Perhaps the most notable difference between the two offerings is that Avance does not provide continuous failover like everRun VM. In the event of a failure, there's a short interval while server instances are transferred from the primary to secondary system. In most cases, though, this results in a momentary lapse in service, but the client remains connected to the server. Thus, most clients will not require rebooting after the switchover. Transfer times were very short; under a second with Linux, and four to five seconds with Windows 2003 Server running Exchange.

Stratus offers two versions of the system: You can purchase software and install it on your own commodity server hardware, or you can opt for the ftServer, which is the Avance software pre-installed on dedicated Stratus hardware. For my test, Stratus provided the Avance software pre-installed on two Dell PowerEdge 1950 servers. However, the installation process is not onerous. It's a simple matter of booting from the installation CD and following the prompts to install the first node, then repeating on the second system. After the two systems have been installed, you log into the Avance management console to configure and register the Avance software.

With the basic setup complete, the next step is installing virtual instances of server operating systems. This is the same procedure followed for XenServer installations in general, including the requirement to install para-virtualization drivers for Windows Server installations, and the method of creating Linux repositories to hold various versions of Linux operating systems.

When you're finished with basic installation and configuration, protecting a server instance is automatic. Any VM running on either node has all of its virtual storage replicated on both physical servers. If a fault occurs, the Avance system migrates the instance to the other physical node without interrupting operation. This process will work if the server's connection to the production network is brought down, if a non-fatal hardware fault (such as one of the two power supplies failing) occurs, or if the server instance itself experiences some kind of fault. The system will not gracefully fail over in the event of a catastrophic hardware failure.

Because Avance does not rely on shared storage, it's substantially less expensive to implement than other solutions. To its credit, it supports the same range of guest operating systems that XenServer does, plus it provides quick recovery from most hardware problems and software faults, without interrupting services to clients for more than a couple of seconds, and without requiring client reboots.

Although Avance does not provide the utterly seamless failover that everRun VM does, it supports a much greater array of guest OSes, has commendable ease of use, boasts an excellent price, and will satisfy most administrators looking for a way to make their virtual applications fault tolerant -- as long as they are willing to run on XenServer.

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