Category: Virtualisation

Virtualisation and cloud computing

Killing Servers with Virtualisation and Swap

The Problem: A problem with virtual machines is the fact that one rogue DomU can destroy the performance of all the others by inappropriate resource use. CPU scheduling is designed to allow reasonable sharing of computational resources, it is unfortunately not well documented, the XenSource wiki currently doesn’t document the “credit” scheduler which is used […]


Xen and Linux Memory Assignment Bugs

The Linux kernel has a number of code sections which look at the apparent size of the machine and determine what would be the best size for buffers. For physical hardware this makes sense as the hardware doesn’t change at runtime. There are many situations where performance can be improved by using more memory for […]



When configuring servers the trade-offs between RAM and disk are well known. If your storage is a little slow then you can often alleviate the performance problems by installing more RAM for caching and to avoid swapping. If you have more than adequate disk IO capacity then you can over-commit memory and swap out the […]


Xen CPU use per Domain

The command “xm list” displays the number of seconds of CPU time used by each Xen domain. This makes it easy to compare the CPU use of the various domains if they were all started at the same time (usually system boot). But is not very helpful if they were started at different times. I […]


A New Strategy for Xen MAC Allocation

When installing Xen servers one issue that arises is how to assign MAC addresses. The Wikipedia page about MAC addresses [1] shows that all addresses that have the second least significant bit of the most significant byte set to 1 are “locally administered”. In practice people just use addresses starting with 02: for this purpose […]


Xen and EeePC

I’ve been considering the possibility of using Xen on an ASUS EeePC as a mobile test platform for an Internet service. While the real service uses some heavy hardware it seems that a small laptop could simulate it when running with a small data set (only a few dozen accounts) and everything tuned for small […]


ISP Redundancy and Virtualisation

If you want a reliable network then you need to determine an appropriate level of redundancy. When servers were small and there was no well accepted virtual machine technology there were always many points at which redundancy could be employed. A common example is a large mail server. You might have MX servers to receive […]

Ha Virtualisation

CPU Capacity for Virtualisation

Today a client asked me to advise him on how to dramatically reduce the number of servers for his business. He needs to go from 18 active servers to 4. Some of the machines in the network are redundant servers. By reducing some of the redundancy I can remove four servers, so now it’s a […]


Hosting a Xen Server

Yesterday I wrote about my search for a hosting provider for a Xen DomU [1]. One response was the suggestion to run a Dom0 and sell DomU’s to other people [2], it was pointed out that Steve Kemp’s project is an example of how to do this well [3]. Unfortunately Steve’s service is full […]


Xen Hosting

I’m currently deciding where to get a Xen DomU hosted. It will be used for a new project that I’m about to start which will take more bandwidth than my current ISP is prepared to offer (or at least they would want me to start paying and serious bandwidth is expensive in Australia). Below is […]