Write barriers have gotten some discussion in the Linux world recently which is a good thing due to a lack of documentation about what write barriers are and when they should be used. Let's first start with the assumption that a user wants file system integrity over performance. This often means turning off write caching on all disks. The reason is that file systems rely on the disks not reordering write requests in order to maintain the integrity of the file system. Of course, turning off write caching harms write performance substantially so another option was invented: write barriers.

I’ve been sharing my thoughts about various virtualization solutions a lot over the last few weeks. Several people have asked me for my opinions on the strengths and weaknesses of various products. I’ve also commented online about these issues. I’ve decided to list some of my observations in order to have something to refer to, for myself and others. I’ve listed options separately for servers and desktops and by order of aptitude to those roles.

I setup a virtual server for a client to host a Rails application on an Ubuntu 7.10 system. Here I have documented the steps to install all the necessary pieces of software. This is a MySQL5.0/Rails2.0/Mongrel1.1/Apache2.2 setup with Subversion for source code management. I realize that Apache is not the most popular frontend but for this client security and maturity are important. I also realize that their are alternatives to Mongrel, MySQL, and Subversion as well that are well worth considering. So rather than this being THE Rails stack I think of it as A Rails stack suited for a particular customer.

Turns out that VirtualBox doesn't support the Ubuntu Server Linux kernel due to it's reliance on PAE support (even in 64-bit apparently). The solution is to run the generic kernel instead. I'm not sure if that can be done when installing Ubuntu Server, or if one has to install Ubuntu desktop and then strip it down. I'll have to do some testing when I get the time.

For more information about what guest operating systems are supported in virtual box:
http://www.virtualbox.org/wiki/Guest_OSes

It’s not recent but SILC server 1.1 was released about 2 months ago. Unfortunately it still doesn’t fix what I see as the biggest issue: that the server cannot be run on a private IP behind NAT. This feature is in the SILC TODO file for features for 1.2 and later. The question is will it arrive sooner than IPv6 adoption that alleviates much of the need for NAT.