Month: August 2008

Links August 2008

Michael Janke is writing a series of posts about estimating availability of systems, here is a link to the introduction [1]. He covers lots of things that people often miss (such as cooling). If you aren’t about to implement a system for reliability then it’s an interesting read. If you are about to implement a […]


Improving Blog Latency to Benefit Readers

I just read an interesting post about latency and how it affects web sites [1]. The post has some good ideas but unfortunately mixed information on some esoteric technologies such as infiniband that are not generally applicable with material that is of wide use (such as ping times). The post starts by describing the latency […]


Swapping to a Floppy Disk

In the mid 90’s I was part-owner of a small ISP. We had given out Trumpet Winsock [1] to a large number of customers and couldn’t convert them to anything else. Unfortunately a new release of the Linux kernel (from memory I think it was 2.0) happened to not work with Trumpet Winsock. Not wanting […]


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 […]


Is SE Linux Unixish?

In a comment on my AppArmor is dead post [1] someone complained that SE Linux is not “Unixish“. The security model in Unix is almost exclusively Discretionary Access Control (DAC) [2]. This means that any process that owns a resource can grant access to the resource to other processes without restriction. For example a user […]


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 […]


Play Machine Downtime

From the 13th to the 14th of August my Play Machine [1] was offline. There was a power failure for a few seconds and the machine didn’t boot correctly. As I had a lot of work to do I left it offline for a day before fixing it. The reason it didn’t boot was that […]


AppArmor is Dead

For some time there have been two mainstream Mandatory Access Control (MAC) [1] systems for Linux. SE Linux [2] and AppArmor [3]. In late 2007 Novell laid off almost all the developers of AppArmor [4] with the aim of having the community do all the coding. Crispin Cowan (the founder and leader of the AppArmor […]


Switches and Cables

I’ve just read an amusing series of blog posts about bad wiring [1]. I’ve seen my share of wiring horror in the past. There are some easy ways of minimising wiring problems which seem to never get implemented. The first thing to do is to have switches near computers. Having 48 port switches in a […]


Purging an RT Database

I had a problem where the email address spammed a Request Tracker (RT) [1] installation (one of the rules for running a vaction program is that you never respond twice to the same address, another rule is that you never respond to automatically generated messages). Deleting these tickets was not easy, the RT web […]