June 2010

2

The Lawnmower Man

Posted on Tuesday, 8 June 2010

Introduction


This website shares a server with various other network services that form the foundation of my online life (i.e. IRC and Email) and I've been running into capacity issues in the last few months, so I'm running an experiment whereby I upgrade to brand new hardware (Quad Core i7, 8GB of RAM) and partition the available resources across virtual machines so the various network services are isolated into logical security zones.

Whining


I have plenty of experience using Xen for this sort of thing, but that's becoming more and more irrelevant in newer kernels/distributions. As much as I think that's a shame and a stupid upstream decision, I can't change it, so I need to move on to KVM and libvirt.

Resolution


So, with the beefy new server booted up in a -server kernel and a big, empty LVM Volume Group I got to work creating some virtual machines. This post is mainly a reminder to myself of the things I need to do for each VM :)

Action


These are the steps I used to make a VM with 1GB of RAM, 10GB / and 1GB of swap:

Create an LVM Logical Volume


lvcreate -L11G -n somehostname VolumeGroup

Create a VM image and libvirt XML definition


ubuntu-vm-builder kvm lucid --arch amd64 --mem=1024 --cpus=1

--raw=/dev/VolumeGroup/somehostname --rootsize=10240 --swapsize=1024

--kernel-flavour=server --hostname=somehostname

--mirror=http://archive.ubuntu.com/ubuntu/ --components=main,universe

--name 'Chris Jones' --user cmsj --pass 'ubuntu' --bridge virbr0

--libvirt qemu:///system --addpkg vim --addpkg ssh --addpkg ubuntu-minimal

Catchy command, huh? ;)

Wait


(building the VM will take a few minutes)

Modify the libvirt XML definition for performance


The best driver for disk/networking is the paravirtualised "virtio" driver. I found that ubuntu-vm-builder had already configured the networking to use this, but not the disk, so I modified the disk section to look like this:
<disk type='block' device='disk'>

  <source dev='/dev/VolumeGroup/somehostname'/>

  <target dev='vda' bus='virtio'/>

</disk>

Modify the libvirt XML definition for emulated serial console


I don't really want to use VNC to talk to the console of my VMs, so I add the following to the <devices> section of the XML definition to make a virtualised serial port and consider it a console:


<serial type='pty'>

  <target port='0'/>

</serial>

<console type='pty'>

  <target port='0'/>

</console>

Modify the libvirt XML definition for a better CPU


I'm running this on an Intel Core i7 (Nehalem), but libvirt's newest defined CPU type is a Core2Duo, so we'll go with that in the root of the <domain> section:
<cpu match='minimum'>

  <model>core2duo</model>

</cpu>

Import the XML definition into the running libvirt daemon
virsh define /etc/libvirt/qemu/somehostname.xml

Mount the VM's root filesystem


The Logical Volume we created should be considered as a whole disk, not a mountable partition, but dmsetup can present the partitions within it, and these should still be present after running ubuntu-vm-builder:

mkdir /mnt/tmpvmroot

mount /dev/mapper/VolumeGroup-somehostnamep1 /mnt/tmpvmroot

Fix fstab in the VM


Edit /mnt/tmpvmroot/etc/fstab and s/hda/vda/

Configure serial console in the VM


Edit /etc/init/ttyS0.conf and place the following in it:


# ttyS0 - getty

#

# This service maintains a getty on ttyS0 from the point the system is

# started until it is shut down again.

start on stopped rc RUNLEVEL=[2345]

stop on runlevel [!2345]

respawn

exec /sbin/getty -L 115200 ttyS0 xterm

Edit /boot/grub/menu.lst and look for the commented "defoptions" line. Change it to:


# defoptions=console=ttyS0 console=tty0

(the default "quiet splash" is not useful for servers IMHO)

Unmount the VM's root filesystem


umount /mnt/tmpvmroot

rmdir /mnt/tmpvmroot

Start the VM


virsh start somehostname

SSH into the VM


I didn't specify any networking details to ubuntu-vm-builder, so the machine will boot and try to get an address from DHCP. By default you'll have a bridge device for libvirt called virbr0 and dnsmasq will be running, so watch syslog for the VM getting its address.
ssh cmsj@192.168.122.xyz

you should now be in your VM! Now all you need to do is configure it to do things and then fix its networking. My plan is to switch the VMs to static IPs and then use NAT to forward connections from public IPs to the VMs, but you could bridge them onto the host's main ethernet device and assign public IPs directly to the VMs.

7

Python decisions

Posted on Thursday, 3 June 2010

Every time I find myself hacking on some Python I find myself second guessing all sorts of tiny design decisions and so I figure the only way to get any kind of perspective on them is to talk about them. Either I'll achieve more clarity through constructing explanations of what I was thinking, or people will comment with useful insights. Hopefully the latter, but this is hardly the most popular blog in the world ;)
So. What shall we look at first. Well, I just hacked up a tiny script last night to answer a simple question:

Is most of my music collection from the 90s?



Obviously what I want to do here is examine the ID3 tags of the files in my music collection and see how they're distributed. A quick search with apt showed that Ubuntu 10.04 has two python libraries for dealing with ID3 tags and a quick play with each suggested that the one with the API most relevant to my interests was eyeD3. After a few test iterations of the script I was getting bored of waiting for it to silently scan the roughly 4000 MP3s I have, so I did another quick search and found a progress bar library.

So that's all of the motive and opportunity established, now let's examine the means to the end. If you want to follow along at home, the whole script is here.



try:

 import eyeD3

 import progressbar as pbar

except ImportError:

print("You should make sure python-eyed3 and python-progressbar \

are installed")

 sys.exit(1)

First off this is the section where I'm importing the two non-default python libraries that I depend on. I want to provide a good experience when they're not installed, so I catch the exception and tell people the Debian/Ubuntu package names they need, and exit gracefully. I rename the progressbar module as I import it just because "progressbar" is annoyingly long as a name, and I don't like doing "from foo import *".

Skipping further on, we find the code that extracts the ID3 year tag:
year = tag.getYear() or 'Unknown'

This is something I'm really not sure about the "correctness" of; One of the reasons I went with the eyeD3 library was that the getYear() method returns None if it can't find any data, but I don't really want to capture the result, then test the result explicitly and if it's None set the value to "Unknown", so I went with the above code which only needs a single line and is (IMHO) highly readable.

This is ultimately the crux of the entire program - we've now collected the year, so we can work out which decade it's from:
if year is not 'Unknown':
year = "%s0s" % str(year)[:3]

If this isn't an unknown year we chop the final digit off the year and replace it with a zero. Job done!

Next up, another style question. Rather than store the year we just processed I want to know how many of each decade have been found, so the obvious choice is a dict where the keys are the decades and the values are the number of times each decade has been found. One option would be to pre-fill the dict with all the decades, each with a value of zero, but that seems redundant and ugly, so instead I start out with an empty dict. This presents a challenge - if we find a decade that isn't already a key in the dict (which will frequently be the case) we need to notice that and add it. We could do this by pre-emptively testing the dict with its has_key() method, but that struck me as annoyingly wordy, so I went with:
try:

 years[year] += 1

except KeyError:

 years[year] = 1

If we are incrementing a year that isn't already in the dict, python will raise a KeyError, at which point we know what's happened and know the correct value is 1, so we just set it explicitly. Seems like the simplest solution, but is it the sanest?

The only other thing I wanted to say is a complaint - having built up the dict I then want to print it nicely, so I have a quick list comprehension to produce a list of strings of the format "19xx: yy" (i.e. the decade and the final number of tracks found for that decade), which I then join together using:
', '.join(OUTPUT)

which I hate! Why can't I do:
OUTPUT.join(', ')

(where "OUTPUT" is the list of strings). If that were possible, what I'd actually do is tack the .join() onto the end of the list comprehension and a single line would turn the dict into a printable string.

So there we have it, my thoughts on the structure of my script. I'd also add that I've become mildly obsessive about getting good scores from pylint on my code, which is why it's rigorously formatted, docstring-ed and why the variable names in the __main__ section are in capitals.

What are your thoughts?

Oh, and the answer is no, most of my music is from the 2000s. The 1990s come in second :)