This page describes various techniques for developers, derivatives, and OEMs to change an Ubuntu system to reduce the required disk footprint.

Contents

  1. Reduce package count
  2. Use XFCE instead of GNOME
  3. Drop unnecessary packages
  4. Drop unnecessary files
    1. Technique
    2. Documentation
    3. Translations
    4. Landscape
  5. Compress files
  6. Drop login manager
  7. Select components of metapackages
  8. Select X.org video driver
  9. Disable apt caches
  10. Trim log files
  11. Tune the filesystem
  12. Explore alternate filesystems
  13. Reconfigure and rebuild the kernel
  14. Remove unnecessary kernel modules
  15. Other potential possibilities

Reduce package count

The most obvious technique to reduce disk space is to strip out all unnecessary software packages from the install image. For example, a small system typically does not require an on-disk office suite, so eliminating OpenOffice.org will result in space savings.

Rather than starting with the default ubuntu-desktop meta-package and stripping it down, it is easier to come from the other direction. Start with the ubuntu-minimal meta-package and slowly add packages until the base requirements are met.

Configuring the image to eliminate packages with mere "Recommends" priority is another important technique.

Use XFCE instead of GNOME

The default Ubuntu installation uses the GNOME desktop environment which has quite a large set of dependencies and related disk space cost.

Xubuntu provides the XFCE4 desktop environment, which is still highly functional and provides ability for customization. A default Xubuntu installation weighs in at 1.6G, versus the 2.1G footprint of Ubuntu.

If you use XFCE4, you can drop libgnome2-0's dependency to gvfs, to remove gvfs, libgdu, and udisks (about 5 MB). This also helps saving some boot time (order of 0.5 s). The patch was applied to maverick's libgnome and can be easily backported to lucid-based systems. On the other hand, if you want a more modern XFCE which stops using the deprecated hal and instead uses udisks and upower, then switch to the xubuntu-dev PPA instead.

Drop unnecessary packages

Drop unnecessary files

Technique

If it is acceptable for the project to not install local documentation, manpages, include files, etc., these can be ignored.

One-time removal during image build (livecd-rootfs in Ubuntu or live-helper for OEM) is not enough, since each package update or additonal application installation would bring back the files. So this requires filtering of those files in dpkg.

This is provided by a dpkg patch which recently got committed upstream and got backported into Ubuntu 10.10. Please see man dpkg for details about those options. The patch can be backported to Ubuntu 10.04 LTS based projects.

Since the original debootstrap will install quite a lot of base packages without dpkg, and the project-config package is not necessarily installed before any other package, an additional cleanup during image builds is still required, though. This should remove the same directories/files as the dpkg filter options. These commands can be put into live-helper's LH_HOOKS configuration, or into some project-config's postinst.

Documentation

Translations

If you use packages from universe, /usr/share/locale/ will have a lot of (probably unneeded) translations. If you only need to support a relatively small subset of languages, the unnecessary ones can be filtered out with above dpkg filters:

If you use XFCE, /usr/share/xfce4/doc has translations; these could also be moved into langpacks.

Landscape

If your project is meant to use landscape, you can remove test suites from Landscape dependencies → /usr/share/pyshared/twisted/test, /usr/share/pyshared/twisted/*/test in the python-twisted-{core,web} packages [3 MB]: 

path-exclude /usr/share/pyshared/twisted/test*
path-exclude /usr/lib/python*/dist-packages/twisted/test*
path-exclude /usr/share/pyshared/twisted/*/test*
path-exclude /usr/lib/python*/dist-packages/twisted/*/test*

Compress files

Drop login manager

For projects which are only supposed to be single-user with autologin, we do not need to install gdm (which weighs 13 MB together with its extra non-XFCE dependencies). Instead, this could be replaced with a simple upstart script which directly launches a session for the user.

Select components of metapackages

It is common for large upstream meta-packages to depend, in turn, on other meta-packages, in order to create a single conceptual package. Examples are texlive which depends on all packages that comprise the TeX life LaTeX distribution, and openoffice.org which depends on all individual components (writer, draw, etc.)

If you only need some parts, only depend on the individual components of those.

Select X.org video driver

Ubuntu ships a metapackage xserver-xorg-video-all which depends on all available (and supported) video drivers, such as xserver-xorg-video-apm, xserver-xorg-video-ati, xserver-xorg-video-vesa, etc.

In a small-disk-footprint scenario, the hardware platform is typically extremely specific. Thus, it is highly unlikely that the final image will need both the nouveau and the ati video drivers, and it will almost certainly not need the very old drivers like sis, s3, or mga. So you can explicitly seed the needed video driver(s) for the particular -desktop task.

Each video driver provides the virtual package xserver-xorg-video-abiversion, which will satisfy xserver-xorg's dependency.

If your project does not use Tasks (as Ubuntu does), but directly installs the project-desktop metapackage, this will still cause -video-all to be pulled in. To fix this, you need to modify the xorg source package and replace the -video-all dependency with something lightweight like -vesa or -fbdev.

The input drivers are packaged similarly, with the xserver-xorg-input-all metapackage. You can explicitly seed a required subset of the drivers and change the -input-all dependency to avoid installing all available drivers. Potential candidates for removal are xserver-xorg-input-synaptics (for touchpads), xserver-xorg-input-vmmouse (mouse in virtual machine guests), and xserver-xorg-input-wacom (tablet/touch screen driver).

Disable apt caches

Apt stores two caches in /var/cache/apt/: srcpkgcache.bin is rather useless these days, and pkgcache.bin is only needed for faster lookups with apt-cache (software-center has its own cache). Removing those two buys 26 MB, for the price of apt-cache taking an extra two seconds for each lookup.

An image build needs to rm /var/cache/apt/*.bin. However, the next apt-get update would bring it back, thus the caches need to be disabled in the configuration. Create a file /etc/apt/apt.conf.d/02nocache with the following contents:

However, please note that if you need to use synaptic, you need to keep pkgcache; see bug 596898.

Trim log files

To avoid piling up too many log files in /var/log/, consider reconfiguring rsyslog to only keep one rotation and rotate daily. This requires a patch to the rsyslog package.

Tune the filesystem

The default Lucid filesystem is ext4, and the default setting for ext4 is to use a 4K blocksize. With this setting, even creating a one-byte file will result in a 4K block allocation. On a system with many small files, this default blocksize will produce much wasted space.

ext4 can be configured to use a 1K block size, which tends to be more space-efficient on a small-disk system. The configuration must occur at partition and filesystem creation time, meaning you must modify the installer in order to pass the correct arguments to mkfs.ext4.

The -T small argument is perfect for tuning ext4 on a small 512M disk. 

# mkfs.ext4 -T small /dev/sda1

To enable this option in your install image, modify your preseed configuration in the following fashion: 

--- install/disk-recipe.orig
+++ install/disk-recipe
@@ -12,6 +12,7 @@
        format{ }
        use_filesystem{ }
        reserved_for_root{ 1 }
+       usage{ small }
        $default_filesystem{ }
        mountpoint{ / } .

Using this technique on an i386 test system reduced the disk footprint to 604M from a 736M footprint using 4K blocks.

More partman documentation can be found here:

However, please note that this severely increases boot time and decreases performance.

Explore alternate filesystems

The Linux world eagerly awaits the stabilization of btrfs. It promises ponies for everyone, including on-the-fly file compression. This feature gives btrfs a huge advantage over squashfs for small systems, since the filesystem is read-write versus read-only.

The Lucid kernel has the technical capability to use btrfs partitions, but it is completely unsupported.

That being said, it is indeed possible to convert your root filesystem to btrfs and boot into it with the default Lucid kernel.

One technique is described in this btrfs rootfs howto. While the basic premise described in the post is sound, it unfortunately does not demonstrate the capability of btrfs data compression, since the existing ext4 data are converted in-place, and are not packed as tightly as they could be otherwise.

One might think that using the btrfsctl utility to defragment the btrfs partition would be the solution, and conceptually, one would be correct. Unfortunately, the version of btrfs-tools in Lucid is not new enough. One cannot simply build the latest btrfs-tools from upstream either, since the userspace tools are tightly coupled with the kernel version.

A working technique is to leave some free space on the drive during installation instead of using the entire disk for the installation. After installation completes and the system reboots, boot into the live image. Create a new btrfs partition in the free space, and mount it with the compress option. Then, copy all the data from the ext4 partition to the btrfs partition: cp -ax is your friend.

You can now point grub at your btrfs partition using the technique described in the howto, and Lucid will boot quite happily.

Using this technique on an i386 test system reduced the disk footprint to 303M from a 736M ext4 footprint.

The btrfs wiki is a useful resource.

Reconfigure and rebuild the kernel

The default i386 Lucid kernel and modules occupy 86M of disk. This space can be reclaimed by only building the modules needed for your target platform.

Even worse, modules are double-counted, since they are included in the initrd as well. Consider building your kernel without an initrd. You'll not only save space, you'll also decrease the boot time as well. Bonus!

Device firmware also needlessly occupies disk space. Be sure to remove the firmware files unnecessary for your platform.

Remove unnecessary kernel modules

If you have not reconfigured the kernel, consider simply removing the unused modules and firmware from your system. Check which modules get loaded (plug in all necessary hardware first so that all drivers get loaded), and then remove all kernel modules from /lib/modules/version which are not listed in lsmod.

Obviously this technique is not easily maintainable.

If you are using live-helper, a hook like this can save you a lot of space.

Other potential possibilities

These do not have a readily made implementation yet, but are worth investigating if needed:

ReducingDiskFootprint (last edited 2012-10-11 13:54:27 by pitti)