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How do I size the physical server to host virtual machines?

I want to convert a physical machine to a virtual machine. How can I select and size the right server infrastructure to host VMs?

The fundamental requirement when sizing a platform to host virtual machines is to have enough resources to meet the VMs' requirements. This is basically the same for any hypervisor: the physical host delivers resources for each VM.

The four 'food groups' for VMs are CPU, RAM, disk and network. Generally, the two performance pain points will be RAM and disk.

Disk has two dimensions: capacity and performance. You need more of both than you think to host VMs. Identify and specify the transactional (IOPS) performance requirements of your storage as well as the throughput (MBps). Also allot spare disk capacity for snapshots used to back up the virtual machines.

Most operating systems use RAM as disk cache, but don't report it as being used. If you don't size your VM environment to accommodate this cache, it can lead to poor application performance. Data centers that convert physical machines to virtual ones will roll back to a physical infrastructure due to this mistake.

The simplest approach to choosing the right infrastructure is to sum the hardware in each physical server you are replacing with VMs. You could buy enough resources to meet the sum of these installed resources, but this tends to be expensive.

Another approach is to watch resource consumption. The better you know your workload, the better you can size your virtualization platform. Identify how much of each resource the physical machine actually consumes on average, and combine these numbers. You should buy enough hardware for the total average usage. Make sure to allow for some overhead, since your hypervisor of choice needs resources to manage the VMs it runs.

Both of these methods assume that there is currently enough resources for each machine. Be mindful of physical machines that perform inadequately and think about which machines need more resources when you virtualize them. Budget for any of these extra resources.

About the author:
Alastair Cooke is a freelance trainer, consultant and blogger specializing in server and desktop virtualization. Known in Australia and New Zealand for the APAC virtualization podcast and regional community events, Cooke was awarded VMware's vExpert status for his 2010 efforts.

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