I've recently had to set up a new machine, but didn't have an install cdrom available, so I decided to use the easiest method for installing Ubuntu; PXE booting. Here's how I did it. PXE involves setting up two simple technologies, DHCP and TFTP. We start by setting up TFTP.

TFTP is Trivial File Transfer Protocol, a cut down version of FTP. There are a number of TFTP servers in Debian and Ubuntu, but not all of them support the extensions that the pxelinux bootloader used by debian-installer need. Experience has shown that tftpd-hpa works correctly, so we'll want to install that.

ace root% apt-get install tftpd-hpa

Note: If this installs an inetd at the same time, you may need to restart the inetd so it enables the tftpd service.

The tftpd will serve files out of /var/lib/tftpboot, so we need to add some files for it to serve. You can use this script to fetch various netboot installers from Ubuntu's servers.

#!/bin/bash

set -u
set -e

cd /var/lib/tftpboot

for dist in dapper feisty gutsy hardy intrepid; do
    mkdir -p $dist
    for arch in amd64 i386; do
        mkdir -p $dist/$arch/
        (cd $dist/$arch/ && ncftpget -RT \
           ftp://archive.ubuntu.com/ubuntu/dists/$dist/main/installer-$arch/current/images/netboot/)
    done
done

Download ubuntu-tftp-update.sh

Now we need to alter our dhcpd configuration. (You are using DHCP aren't you?) All we need to add is a group declaration to your subnet declaration, adding a next-server and a filename parameter. You can then add a host declaration for any machine you want to netboot into the installer.

group { # intrepid amd64
     next-server 10.0.0.1;
     filename "intrepid/amd64/pxelinux.0";
     host foobar { hardware ethernet 00:22:15:45:cc:fa; fixed-address foobar.example.com; }
}

You'll need to restart the dhcp server so it picks up the new setting. The next-server parameter is the name or IP address of your tftp server. filename is the path to the bootloader. Obviously, you can use this to pick which version of the installer you want to run. If you do a lot of installations, it might be worth configuring every installer you're likely to use and then move hosts in and out of the suitable group as and when you need to install them.

All that's left to do now is to boot the computer and set it to boot from the network and enjoy medialess installation.

Dear Lazyweb,

We have a web application that has quite a large database and reasonable usage. Back in the dim and distant past, we scaled the application by the age-old method of using several read-only slave databases to prevent reads on the master swamping writes. This worked well for several years, and then we introduced memcached into the mix to improve performance by reducing the number of reads from the database. This improved our database capacity even further.

Now the question has arisen about reducing or even removing the code to read from the slaves. I'm trying to come up with some compelling reasons to keep the application reading from the slaves. The pros and cons I currently have for removing the code are:

Pros

• Reduces code complexity
• Removes consistency problems due to latency in the replication. This is less of a problem than it used to be after we solved a problem with our replication

Cons

• Reduces our existing capacity
• Cache flushes would cause huge spikes on our master server until the cache filled up again
• Caches wouldn't help queries with unique critera

I would appreciate any additional reasons, pro or cons. We already have an existing non-live slave for backups and slow queries by developers. We would retain a slave for redundancy in the case of master failure. I'm only looking for issues that would affect the application.