Lately I've been wanting to replace the hard drive in my laptop with a nice fast second generation Intel X25-m. Unfortunately my disk footprint is just a little too high to comfortably fit on the disk I want to buy, so I have been looking to free up a little space wherever I can.

What I Started With

I had three test VirtualBox images totaling around 8.5 GB. I figured that since two of the images were running the same version of Ubuntu that QEMU's qcow2 copy-on-write disk images would be able to save me a bit of room. My original three machines looked like this:

total 8.5G
3.8G OpenVZ 1 Test.vdi
2.8G OpenVZ 2 Test.vdi
1.9G patshead.com Test.vdi

The first two were Ubuntu 8.04 servers, the last was running Ubuntu 9.04. I decided that it would be easiest to rebuild these machines from scratch and migrate over whatever data was needed.

Creating The Base Disk Images

In order to keep organized I created a pair of directories to keep disk images in. One for the read only base images, and another for the writable images:

~/qemu/qcow-ro
~/qemu/qcow-rw

I created two temporary disk images to hold fairly stripped down installations of Ubuntu Hardy and Jaunty server. I created the initial machines without swap space. I also made sure to install all the latest updates and remove any extra outdated kernel packages (they are surprisingly large!). I also made sure to install any software that I knew I would want to have available in all my future machines.

Once everything was installed I needed to clean things up, and I think I made a small error here. I ran apt-get clean to remove any downloaded packages. I probably should have zeroed out the files with a command like shred -n 1 -z /var/cache/apt/archives/*.deb instead.

I also made sure to delete the ssh host keys (rm /etc/ssh/ssh_host_*). When I boot a new image I make sure to run dpkg-reconfigure openssh-server to generate a new set of keys for the new server.

At this point I was left with two images that needed to be shrunk down. The compression of qcow2 images is only applied when the image is created, any later writes to the same disk image will be uncompressed. I used qemu-img convert to recompress the images.

This left me with two smaller base images:

total 834M
438M hardy-root.qcow2
397M jaunty-root.qcow2

From here I just had to create more qcow2 images using one of these two as the base. Specifying the full path for the base_image seemed to be important, for example:

qemu-img create -b ~/qemu/qcow-ro/hardy-root.qcow2 -f qcow2 ~/qemu/qcow-rw/new-hardy-root.qcow2

To save a bit more space, I created an empty swap disk image and ran mkswap on it. The scripts I wrote for starting my QEMU machines will make a copy of my empty swap image as needed. They are deleted when the machine shuts down.

Squeezing Out a Bit More Space

After I finished loading all the data into my test images I decided to try to recompress the images using qemu-img convert. In one case, I save about 200 meg (which was about 40%). In the most cases, the images got bigger! Zeroing out files instead of deleting them probably would have helped quite a bit in this case as well.

What I Ended Up With

After setting up equivalents of my original three machines my disk footprint was just under 2 GB. I was very happy with that result. I have added another server and I am now right around 2.5 GB. Here are the images I am currently using:

qcow-ro:
total 834M
438M hardy-cow-root.qcow2
397M jaunty-cow-root.qcow2

qcow-rw:
total 1.6G
 28K EMPTYSWAP.qcow2
757M OpenVZ-Test-1-root.qcow2
348M OpenVZ-Test-2-root.qcow2
3.0M Patshead-dev-root.qcow2
517M movabletype-root.qcow2
 28K movabletype-swap.raw

Overall, I am very happy with the savings in disk space. It is also less costly in both time and disk space to throw up a temporary test machine if I need one.

The biggest drawback is that QEMU, even with the KQEMU accelerator, is quite a bit slower than Virtualbox or VMware Server. Fortunately, the performance is more than acceptable for what I'm using it for. Your mileage may vary, of course.

Now I just have to wait for the G2 X25-m drives to ship again...

See also:

• How I Reduced My Virtual Machine Disk Images By Over 75% With QEMU
• Experimenting with Compressed Swap
• Fix for Karmic Koala 64 bit Flash Plugin
• Saving Space With fusecompress
• Using QR Codes for Hardcopy Backups of Private Keys
• Achieving Better Compression with lrzip and rzip
• Testing btrfs Root File System on Ubuntu
• Automatically Configuring Multiple Heads with x.org
• Thoughts After Running btrfs for a Month
• Simple Automated btrfs Snapshots with btrfs-snap
• Linux Kernel 2.6.36 on the HP Pavilion DV8T
• Tweak btrfs-snap for More Frequent Snapshots
• My Backup Strategy for My Laptop
• btrfs - Six Months Later
• Disable the 'Resize Window Grippers' in Ubuntu 11.04 Natty Narwhal
• My First btrfs Failure and the State of Ubuntu's btrfs Support
• Supplimenting btrfs-snap With apt-btrfs-snapshot