5.12. Allocating disk space

5.12.1. Partitioning schemes

When it comes to partitioning your machine, there is no universally correct way to do it. There are many factors that must be taken into account depending on the purpose of the machine.

For a simple workstation with limited disk space, such as a laptop, you may have as few a 3 partitions. A partition for /, /boot, and swap. However, for most users this is not a recommended solution.

The traditional way is to have a (relatively) small root filesystem, and separate partitions for filesystems such as /usr and /home>. Creating a separate root filesystem if the root filesystem is small and not heavily used, it is less likely to become corrupt when the system crashes, and therefore make it easier to recover a crashed system. The reason is to prevent having the root filesystem get filled and cause a system crash.

When creating your partitioning scheme, there are some things you need to remember. You cannot create separate partitions for the following directories: /bin, /etc, /dev, /initrd, /lib, and /sbin. The contents of these directories are required at bootup and must always be part of the / partition.

It is also recommended that you create separate partitions for /var and /tmp. This is because both directories typically have data that is constantly changing. Not creating separate partitions for these filesystems puts you at risk of having log file fill up our / partition.

An example of a server partition is:
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda2             9.7G  1.3G  8.0G  14% /
/dev/hda1             128M   44M   82M  34% /boot
/dev/hda3             4.9G  4.0G  670M  86% /usr
/dev/hda5             4.9G  2.1G  2.5G  46% /var
/dev/hda7              31G   24G  5.6G  81% /home
/dev/hda8             4.9G  2.0G  670M  43% /opt

The problem with having many partitions is that it splits the total amount of free disk space into many small pieces. One way to avoid this problem is to use to create Logical Volumes.

5.12.2. Logical Volume Manager (LVM)

Using LVM allows administrators the flexibility to create logical disks that can be expanded dynamically as more disk space is required.

This is done first by creating partitions with as an 0x8e Linux LVM partition type. Then the Physical Partitions are added to a Volume Group and broken up into chunks, or Physical Extents Volume Group. These extends can then be grouped into Logical Volumes. These Logical Volumes then can be formatted just like a physical partition. The big difference is that they can be expanded by adding more extents to them.

Right now, a full discussion of LVM is beyond the scope of this guide. However, and excellent resource for learning more about LVM can be found at http://www.tldp.org/HOWTO/LVM-HOWTO.html.

5.12.3. Space requirements

The Linux distribution you install will give some indication of how much disk space you need for various configurations. Programs installed separately may also do the same. This will help you plan your disk space usage, but you should prepare for the future and reserve some extra space for things you will notice later that you need.

The amount you need for user files depends on what your users wish to do. Most people seem to need as much space for their files as possible, but the amount they will live happily with varies a lot. Some people do only light text processing and will survive nicely with a few megabytes, others do heavy image processing and will need gigabytes.

By the way, when comparing file sizes given in kilobytes or megabytes and disk space given in megabytes, it can be important to know that the two units can be different. Some disk manufacturers like to pretend that a kilobyte is 1000 bytes and a megabyte is 1000 kilobytes, while all the rest of the computing world uses 1024 for both factors. Therefore, a 345 MB hard disk is really a 330 MB hard disk.

Swap space allocation is discussed in Section 6.5.

5.12.4. Examples of hard disk allocation

I used to have a 10 GB hard disk. Now I am using a 30 GB hard disk. I'll explain how and why I partitioned those disks.

First, I created a /boot partition at 128 MG. This is larger than I will need, and big enough to give me space if I need it. I created a separate /boot partition to ensure that this filesystem will never get filled up, and therefore will be bootable. Then I created a 5 GB /var partition. Since the /var filesystem is where log files and email is stored I wanted to isolate it from my root partition. (I have had log files grow overnight and fill my root filesystem in the past.) Next, I created a 15 GB /home partition. This is handy in the event of a system crash. If I ever have to re-install Linux from scratch, I can tell the installation program to not format this partition, and instead remount it without the data being lost. Finally since I had 512 MG of RAM I created a 1024 MG (or 1 GB) swap partition. This left me with roughly a 9 GB root filesystem. I using my old 10 GB hard drive, I created an 8 GB /usr partition and left 2 GB unused. This is incase I need more space in the future.

In the end, my partition tables looked like this:

Table 5-3. My Partitions

9 GBroot filesystem
1 GBswap partition
5 GB/var filesystem
15 GB/home filesystem
8 GB/usr filesystem
2 GBscratch partition

5.12.5. Adding more disk space for Linux

Adding more disk space for Linux is easy, at least after the hardware has been properly installed (the hardware installation is outside the scope of this book). You format it if necessary, then create the partitions and filesystem as described above, and add the proper lines to /etc/fstab so that it is mounted automatically.

5.12.6. Tips for saving disk space

The best tip for saving disk space is to avoid installing unnecessary programs. Most Linux distributions have an option to install only part of the packages they contain, and by analyzing your needs you might notice that you don't need most of them. This will help save a lot of disk space, since many programs are quite large. Even if you do need a particular package or program, you might not need all of it. For example, some on-line documentation might be unnecessary, as might some of the Elisp files for GNU Emacs, some of the fonts for X11, or some of the libraries for programming.

If you cannot uninstall packages, you might look into compression. Compression programs such as gzip or zip will compress (and uncompress) individual files or groups of files. The gzexe system will compress and uncompress programs invisibly to the user (unused programs are compressed, then uncompressed as they are used). The experimental DouBle system will compress all files in a filesystem, invisibly to the programs that use them. (If you are familiar with products such as Stacker for MS-DOS or DriveSpace for Windows, the principle is the same.)

Another way to save space is to take special care when formatting you partitions. Most modern filesystems will allow you to specify the block size. The block size is chunk size that the filesystem will use to read and write data. Larger block sizes will help disk I/O performance when using large files, such as databases. This happens because the disk can read or write data for a longer period of time before having to search for the next block. The