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VMware and clustering

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A discussion of clustering isn't complete without talking about VMware, which is becoming a standard in both large and small data centers. The latest version, VMware Infrastructure 3 (VI3), has some interesting clustering characteristics. This article discusses the virtualization of an application (via its IP address); VMware's approach is to virtualize the operating system along with the application. The entire virtual machine (VM) is actually packaged into a couple of physical files on the storage device. Today, with the ability to share storage among multiple servers, any VMware Server (ESX Server) that has access to those files can use VMotion technology to start the VM.

When a user sends a request, something (an application, operating system or server) must answer the request at the correct IP address running the correct application with access to the appropriate data. VMotion does this at the operating system level rather than at the application level. In a VMotion scenario, two ESX Servers have physical access to the VM files on a storage array. Either ESX Server could run the VM and answer the user request.

When a system administrator initiates a VMotion migration, VMware begins to log all activity against the machine. It also has to move a memory map of the first machine to the second machine. With the memory map migrated and the logging applied to the changes on disk, the second machine can now adopt the IP address and the application has moved. If this were a planned migration, the application moves without incident. If it's a true failure on the first server, the memory map is missing. The ability to successfully start the VM and subsequent application is dependent on the resilience of both to a complete power failure.

Today, many of the larger ERP vendors don't support running their application within a virtual session. In addition, many of the larger ERP vendors recommend a full 64-bit server architecture for the ERP database server. VMware isn't available for those platforms. But as CPU power becomes more plentiful (quad-core is around the corner), we may see a shift of ERP database servers running in a VMware cluster. Combined with a continuous mapping of memory between two ESX Servers, this may become a more accepted method of clustering ERP databases.

Database, system, storage and network admins play a part in increasing the availability of an application. Clustering typically refers to the leveraging of duplicate infrastructures to increase the availability of an application or a group of applications.

When a user requests data about an order that was placed in the ERP system, as long as the right application is operating at the requested IP address with access to the data, the request will be acknowledged and serviced. Clustering software like Hewlett-Packard (HP) Co.'s MC/ServiceGuard, IBM Corp.'s High-Availability Cluster Multiprocessing (HACMP) and Microsoft Corp.'s Windows Compute Cluster Server 2003 allow a server to have multiple IP addresses. In a clustered environment, the mission-critical ERP application will have an IP address that's independent from the hardware IP address. Users send requests to an application's IP address, not to the server's IP address. By separating--or virtualizing--the application's IP address from the hardware, the need to have a specific server running to keep the application available is eliminated (see "VMware and clustering,"). This is the first step to ensure that the database layer isn't a single point of failure for the mission-critical application.

This was first published in February 2007

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