When it comes to getting technical support for an application, so long as the software vendor supports running its app in a virtual environment, hardware-related compatibility issues can be avoided, though not entirely eliminated because of the relationship among applications and consolidated storage devices, such as iSCSI and SAN subsystems. These still require specific drivers and support in the virtualization environment.
This virtual machine mobility also makes it easier for IT to migrate an application from one server to another, should performance become a problem. This capability helps guard against changes in application performance as user demand increases or diminishes over time. Say a given server is running four VMs, but user demand on an application running in one of those VMs increases to the point that the server has difficulty supporting the demand. IT can migrate that one application to a more robust system or migrate other VMs to other systems to free up resources.
The cost of this approach is the increased overhead of managing VM images. To have a truly portable environment, in which VMs can be easily moved from one system to another--or recovered in the event of a catastrophic hardware failure--companies must invest in specialized tools, such as Altiris' Deployment Solution. In addition, they must be mindful of keeping images of VMs up-to-date, and understand network and storage topologies so that when a VM is migrated to another server, it has efficient access to storage subsystems.
By creating virtual hardware instances, ESX Server lets IT create multiple instances of OSs on the same box. The tricky part is finding the right mix of applications to run in those VM instances so that users don't see a drop in application performance, all while squeezing the most out of your hardware. The biggest challenge we saw in our tests came when we tried to determine how much application performance on one virtual machine instance suffered when we added other VMs, each running their own applications, to the mix.
Although performance is going to vary by application and the kind of resources the app demands--disk versus memory versus processor--we can make some recommendations: Anticipate the overhead of virtualization. Don't expect virtual hardware resources to deliver much benefit when compared with real hardware. Group apps with different resource requirements on the same hardware. Use VMware's utilities that prioritize resource usage. Employ VMware's and third-party tools that make it easy to migrate VMs to different hardware.
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