8.1.4.3.1. Introduction

There are different types of techniques to gain more disk space, such as sharing of space, freeing unused or redundant space, filesystem tuning and compression. Note: some of these techniques use memory instead of space. As you will see, there are many small steps necessary to free some space.

8.1.4.3.2. Techniques

• Stripping: Though many distributions come with stripped binaries today it is useful to check this. For details see man strip. To find every unstripped file you can use the file command or more convenient the tool findstrip (find it in the Debian perforate package) . Attention: don't strip libraries, sometimes the wrong symbols are removed due to a bad programming technique.

  A recommendation from Russell Marks <rus at beeb.net>:

  These days a lot of people compile with -g, which I find a bit annoying (though AFAIK this only loses you disk space, in practice).

  strip has a --strip-debug option which doesn't strip symbols, but does still get rid of the debugging stuff. This is almost as good, in many cases, and it's ok to use it on libraries. As it happens, I recently got SuSE 6.3, so I can give you a live example:

  bash-2.03# cd /lib bash-2.03# ls -l libc.so.6 -rwxr-xr-x 1 root root 4223971 Nov 6 16:22 libc.so.6 bash-2.03# strip --strip-debug libc.so.6 bash-2.03# ls -l libc.so.6 -rwxr-xr-x 1 root root 1200355 Dec 8 00:13 libc.so.6

• Sparse files: in the case when files contain blocks with only null characters, these blocks can be replaced by lseek command calls, saving space. Applications (and not the operating system) are responsible for creating or handling such files in a correct way. For example, both the tar and cp commands have options for handling sparse files in a correct way. See the this page for more details and for a sparse command to create such files. In Debian, you can also use the zum command from the perforate package.

• Remove Odd Files and Duplicates: Check your system for core files, emacs recovery files <#FILE#> vi recovery files <FILE>.swp, RPM recovery files <FILE>.rpmorig and patch recovery files. Find duplicates, you may try finddup. Choose a system to name your backup, temporary and test files, e.g. with a signature at the end.

• Clean Temporary Files: , e.g. /tmp, there is even a tool tmpwatch.

• Shorten the Log Files: usually the files in /var/log. There are some nice helpers for this task around, e.g. savelog .

• Remove Files: Remove files which are not "necessary" under all circumstances such as man pages, documentation /usr/doc and sources e.g. /usr/src .

• Unnecessary Libraries: You may use the binstats package to find unused libraries (Thanks to Tom Ed White).

• Filesystem: Choose a filesystem which treats disk space economically e.g. rsfs aka Reiser Filesystem. Tune your filesystem e.g. tune2fs. Choose an appropriate partition and block size.

• Reduce Kernel Size: Either by using only the necessary kernel features and/or making a compressed kernel image bzImage.

• Compressed read-write filesystems:

  - There was a project to add compression support to ext2 filesystems: e2compr. This project has a very limited usefulness as it is currently inactive and only supports very old 2.4 and 2.6 kernel versions. No chance to get a working patch on the latest 2.4 and 2.6 releases.

  - Fortunately, a very serious solution exists in the Reiser4 filesystem which includes a compression plugin. Using compression is even reported to be faster than not using it. Actual results should of course vary with the CPU and disk speed, and results might be disappointing on old machines.

• Compressed read-only filesystems

  - The idea is store parts of the system (typically executables and libraries through the /bin, /sbin and /usr directories) in a separate, compressed and read-only filesystem. This is great for saving space and for protecting the system from extern attacks. However, the constraint is that software upgrades can no longer be done when the filesystem is mounted. The whole filesystem contents must be rebuilt and replaced at once.

  - CramFS was the first implemented solution, used by many embedded system makers. Though still maintained, it is dramatically outperformed by SquashFS.

  - SquashFS is the solution everyone should use now. Unlike CramFS, it has no limitations on filesystem and file size, and it achieves much better compression and read speed (up to 5 times!). On slow storage (like USB flash drives), replacing ext2 with SquashFS resulting in a 50% reduction on system boot time (observed in a Linux demo developped by Free Electrons).

• Partition Sharing: You may share swap-space (see Swap-Space-HOWTO) or data partitions between different OS (see mount).

• C library: use uClibc instead of the default GNU C library aka glibc. It takes approximately 400 KB instead of 1700 (glibc). It used by many embedded Linux projects as well as by several tiny Linux distributions. It should satisfy most needs, as the whole Debian 3.0 was ported to it. Caution: if you replace the C library, all applications also need to be recompiled with a dedicated gcc toolchain.

• Busybox - A toolbox implementing most Unix commands. It takes at most 500 KB instead of approximately 10-30 MB with GNU implementations! It is used by almost all embedded Linux projects and small Linux distros. Most commands are implemented, even vi, wget, a dhcp server and client, and even a http server sufficient for most needs. Even if some rare command options are not implemented, even an experienced Unix user hardly makes the difference with GNU commands!

• Kernel: If your needs are fitted with an older kernel version, you can save some space.

  However, with old kernels, you will miss cool features (such as real-time preemption, making your system much more responsive) and support for recent peripherals. It is true that the Linux kernel accumulated bloat over the years: more features, support for more special cases... Now that Linux is taking an increasingly dominant place in embedded systems, work is being done to control the kernel size. The LinuxTiny project releases a set of patches to the Linux kernel sources. In the Configure Standard Kernel Features for Small Systems kernel configuration section, you can unselect features you do not need in a simple computer. The minimum compressed size you can achieve for a Linux 2.6 kernel for a simple PC is approximately 350 KB. Note that more and more LinuxTiny changes are now available in the standard Linux kernel (no patching necessary).

  With the latest kernel releases, you may also try the real-time preemption patches from Ingo Molnar. When full preemption is enabled, it adds unprecedented responsiveness to the Linux kernel. The latency is much reduced, suitable for real real-time systems (less than 100 us!). It is definitely worth trying on old systems to give them a new youth and improve the user experience!

• GUI: Avoid as much Graphical User Interface (GUI) as possible.

• Tiny Distributions: There are some distributions available which fit from one 3.5" floppy to 10MB disk space and fit for small memories, too. See Laptop-HOWTO

8.1.4.5.1. Related HOWTOs

• Small-Memory-mini-HOWTO by Todd Burgess

• 4MB Laptop-HOWTO by Bruce Richardson.

• Linux Loadable Kernel Module HOWTOby Bryan Henderson.

• Kerneld-mini-HOWTO by Henrik Storner.

8.1.4.5.2. Techniques

Check the memory usage with free and top.

You may also reduce the kernel size as much as possible by removing any feature which is not necessary for your needs and by modularizing the kernel as much as possible.

Also you may shutdown every service or daemon which is not needed, e.g. lpd, mountd, nfsd and close some virtual consoles. Please see the Small-Memory-mini-HOWTO for details.

And of course use swap space, when possible.

If possible you may use the resources of another machine, for instance with X, VNC or even telnet. For more information on Virtual Network Computing (VNC), see VNC .