Virtualisation is a method of running multiple independent virtual operating systems on one computer. Here is how to migrate a physical server to a virtual version.
What is virtualisation and why use it?
Virtualisation is a method of running multiple independent virtual operating systems on a single physical computer. It is a way of maximising physical resources to maximise the investment in hardware. Since Moore's law has accurately predicted the exponential growth of computing power, and hardware requirements for the most part have not changed to accomplish the same computing tasks, it is now feasible to turn a very inexpensive 1U dual-socket dual-core commodity server into eight or even 16 virtual servers that run 16 virtual operating systems.
Virtualisation technology is a way of achieving higher server density. However, it does not actually increase total computing power; it decreases it slightly because of overhead. But since a modern US$3,000 2-socket 4-core server is more powerful than a US$30,000 8-socket 8-core server was four years ago, we can exploit this newly found hardware power by increasing the number of logical operating systems it hosts. This slashes the majority of hardware acquisition and maintenance costs that can result in significant savings for any company or organisation.
When to use virtualisation
Virtualisation is the perfect solution for applications that are meant for small- to medium-scale usage. Virtualisation should not be used for high-performance applications where one or more servers need to be clustered together to meet performance requirements of a single application because the added overhead and complexity would only reduce performance. We're essentially taking a 12 GHz server (four cores times three GHz) and chopping it up into 16 750 MHz servers. But if eight of those servers are in off-peak or idle mode, the remaining eight servers will have nearly 1.5 GHz available to them.
While some in the virtualisation industry like to tout high CPU utilisation numbers as an indication of optimum hardware usage, this advice should not be taken to the extreme where application responsiveness gets excessive. A simple rule of thumb is to never let a server exceed 50 percent CPU utilisation during peak loads; and more importantly, never let the application response times exceed a reasonable SLA (Service Level Agreement). Most modern servers being used for in-house server duties are utilised from 1 to 5 percent CPU. Running eight operating systems on a single physical server would elevate the peak CPU utilisation to around 50 percent, but it would average much lower since the peaks and valleys of the virtual operating systems will tend to cancel each other out more or less.
While CPU overhead in most of the virtualisation solutions available today are minimal, I/O (Input/Output) overhead for storage and networking throughput is another story. For servers with extremely high storage or hardware I/O requirements, it would be wise to run them on bare metal even if their CPU requirements can be met inside a Virtual environment. Both XenSource and Virtual Iron (which will soon be Xen Hypervisor based) promise to minimise I/O overhead, yet they're both in beta at this point, so there haven't been any major independent benchmarks to verify this.
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