VM Server Hardware

Oracle VM Server

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which hardware options to purchase for their virtual hosts. In this tip, I share things I've learned when choosing servers for different types of virtual machines and to fit the current and future needs of a virtual environment.

You may be able to fit as many as 100 VMs on a single host, or as few as two. The types of applications that you are running on your virtual machines will largely dictate how many you can put on your host server: for example, servers that have very light resource requirements, such as web, file and print servers, versus servers with medium to heavy resource requirements, such as SQL and Exchange servers. Overall, one should analyze the performance usage of a current environment to get a better understanding of the virtual environment requirements.

Four criteria for sizing up host servers

There are four major criteria to consider when sizing up virtual server hardware: memory, CPU, network and disk resources. Let's start with memory which is typically used up on host servers first.

Memory

When it comes to figuring out how much RAM to put in a host server, I would recommend installing the maximum amount if possible.

Yet, the opposite mentality should be taken when it comes to allocating memory for virtual servers, by only giving a VM the exact amount of memory it needs. Usually with physical servers, more memory than what is needed is installed and much of it ends up being wasted. With a VM, it is simple to increase the RAM at any time, so start out with the minimum amount of memory that you will think it will need and increase it later if necessary. It is possible to over-commit memory to virtual machines and assign more RAM to them then the physical host actually has. By doing this you run the risk of having your VMs swapping to disk when the host memory is exhausted which can cause decreased performance.

CPU

With the advent of multi-core CPUs, it has become easier and cheaper to increase the number of CPUs in a host server. Nowadays, almost all servers come with two or four cores per physical CPU. A good rule of thumb is that four single CPU VMs can be supported per CPU core. This can vary by as much as 1-2 per core, and up to 8-10 per core based on the average CPU utilization of applications running on VMs.

A common misconception with virtual server hardware is that a VM can utilize as much CPU megahertz as needed from the combined total available. For example, a four CPU, quad core 2.6 GHz would have a combined total of 20, 800 megahertz (8 x 2.6 GHz). A single vCPU VM however, can never use more megahertz then the maximum of one CPU/core. If a VM has 2 vCPUs, it can never use more megahertz than the maximum of each CPU/core. How many cores needed will also depend on whether multiple vCPU VMs are used or not.

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