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VMware: The Virtualization Drag: Page 8 of 13

Furthermore, as app loads shift, performance characteristics change as well, and you may find it necessary to remove a VM instance from a given server. Even with that instance gone, the newly freed drive space can't be reallocated to the VMs that have increased performance demands. Unused space will sit empty on the server.

Based on our test results, it's fair to say you shouldn't expect excellent application performance when running multiple disk-bound applications on the same server, unless each application's data storage is off-loaded to a dedicated storage subsystem, such as a SAN. For example, we paired VMs running Microsoft's Exchange Server 2003 and SQL Server 2005 on the same hardware and saw dramatically different results when running tests that had both applications stressing the drive subsystem versus tests where Exchange stressed the disk subsystem and SQL Server ran largely in memory.

Bottom line, think about storage if you plan to dynamically move VMs around. You could remove a VM because an application is no longer supported, but migrating another application to that server might not be an option if the image is too big.

Balancing Act

The big question IT will likely face when moving to ESX Server: How to deploy critical apps virtually without sacrificing too much performance. Fortunately, ESX Server has tools that let you do just that--prioritize application performance by segmenting hardware resources for one VM over another. In our tests we saw that this capability can help deliver on service-level requirements.