Servers

HADR Brings Resiliency to Virtualization

Marathon Technologies everRun VM 4

Marathon's everRun has been well regarded as a server failover add-on for quite a while now, and the company has a good reputation. everRun VM adds similar functionality to Citrix's XenServer virtualization platform, making it competitive with VMware HA.

everRun has a key advantage over VMware's HA, however: When VMware HA fails over, there's a short delay with interruption of service to clients. everRun VM, however, creates two systems that share a single IP address, so that there are no interruptions if the primary server fails.

everRun VM offers some other benefits over VMware HA, including a completely automated install that includes Citrix's XenServer. This feature makes it substantially easier to get up and running compared to VMware with the HA option. It can check for changes to the target servers to ensure there's sufficient processing power and other resources before failover. It also monitors all hardware components, rather than simply a heartbeat connection. Finally, it works with a SAN environment -- but doesn't require one as VMware HA does.

After the XenServer hosts and the XenCenter management workstation are installed, getting everRun VM installed onto each XenServer host is straightforward: You run the installer from the XenServer console, just like any Linux application. The installer bears a striking resemblance to Novell's old C-Worthy interface, which is to say text-based graphics circa 1993 -- but it works. After everRun VM is installed, you will also need to install XenServer tools on each instance of Windows 2003 that you want to protect. (Notably, whereas XenServer supports both Linux and Windows guest operating systems, everRun supports only Windows 2003 instances.)

In addition to the XenCenter console, you will also need to run the everRun Availability Center, for creating protected server instances on a Flash-enabled browser. You can get XenServer installation images, documentation, and licenses direct from Marathon if you're not already using it.

The recommended physical setup between the two servers running XenServer and everRun VM requires four NIC connections: one for the production network, one for the management network, and two for heartbeat connections, using crossover cables. (This is not required but strongly recommended because connecting them through a switch or router introduces a single point of failure). Storage can be local, direct attached, or SAN, either iSCSI or FC.

After everything is configured, using the everRun Availability Center to create protected server instances is simple. You just choose an instance of Windows 2003 on either XenServer system and tell everRun to protect it. everRun VM will clone the instance to the other XenServer system. Both instances have the same IP address, even the same MAC address, and are kept synchronized over the heartbeat cables. If the primary server instance fails, or if storage, network adapters, or other hardware fails, the secondary server takes over, completely imperceptible to the user.

The process of managing a protected copy of Windows remains the same. Any action taken on the primary copy is duplicated automatically on the second copy. It is possible to protect server instances on both XenServer systems: You can protect a primary instance or multiple instances on the first, which are duplicated on the second, and one (or more) instances on the second, which are duplicated on the first. Of course, care should be taken to ensure that if one physical server fails that the other won't be overloaded. After a failure is repaired, everRun automatically resynchronizes the two instances; there is no need for administrator intervention.

everRun VM is limited in scope, currently supporting virtualization of Windows 2003, and only on XenServer. However, what it does, it does very well, providing the highest level of fault tolerance and great ease of use. At $2,000 per physical XenServer, or $4,500 for a license for both XenServer and everRun VM, it is reasonably priced for a continuous high availability solution.

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