Partitioning your disk simply refers to the act of breaking up your disk into sections. Each section is then independent of the others. It's roughly equivalent to putting up walls in a house; if you add furniture to one room it doesn't affect any other room.
If you already have an operating system on your system (Windows95, Windows NT, OS/2, MacOS, Solaris, FreeBSD) and want to stick Linux on the same disk, you will probably need to repartition the disk. In general, changing a partition with a filesystem already on it will destroy any information there. Thus you should always make backups before doing any repartitioning. Using the analogy of the house, you would probably want to move all the furniture out of the way before moving a wall or you risk destroying it. Luckily, there is an alternative for some users; see Lossless Repartitioning When Starting From DOS, Win-32 or OS/2, section 4.6.
At a bare minimum, GNU/Linux needs one partition for itself. You can have a single partition containing the entire operating system, applications, and your personal files. Most people feel that the swap partition is also a necessity, although it's not strictly true. ``Swap'' is scratch space for an operating system, which allows the system to use cheap disk storage as ``virtual memory''. By putting swap on its own partition Linux can make much more efficient use of it (it is possible to force Linux to use a regular file as swap, but it is not recommended).
Most people choose to give GNU/Linux more than the minimum number of partitions, however. There are two reasons you might want to break up the filesystem into a number of smaller partitions. The first is for safety. If something happens to corrupt the file system, generally only one partition is affected. Thus, you only have to replace (from the backups you've been carefully keeping) a portion of your system. At a bare minimum, you should consider creating what is commonly called a ``root partition''. This contains the most essential components of the system. If any other partitions get corrupted, you can still boot into GNU/Linux to fix the system. This can save you the trouble of having to reinstall the system from scratch.
The second reason is generally more important in a business setting,
but it really depends on your use of the machine. Suppose something
runs out of control and starts eating disk space. If the process
causing the problem happens to have root privileges (the system keeps
a percentage of the disk away from users), you could suddenly find
yourself out of disk space. This is not good as the OS needs to use
real files (besides swap space) for many things. It may not even be a
problem of local origin. For example, getting spammed with e-mail can
easily fill a partition. By using more partitions, you protect the
system from many of these problems. Using mail as an example again, by
putting /var/spool/mail on its own partition, the bulk of the
system will work even if you get spammed.
Another reason applies to you only if you have a large IDE disk, and are using neither LBA addressing, nor overlay drivers (sometimes provided by hard disk manufacturers). In this case, you will have to put the root partition into the first 1024 cylinders of your hard drive (usually around 524 megabytes).
The only real drawback to using more partitions is that it is often difficult to know in advance what your needs will be. If you make a partition too small then you will either have to reinstall the system or you will be constantly moving things around to make room in the undersized partition. On the other hand, if you make the partition too big, you will be wasting space that could be used elsewhere. Disk space is cheap nowadays, but why throw your money away?
PC BIOS generally adds additional constraints for disk partitioning. There is a limit to how many ``primary'' and ``logical'' partitions a drive can contain. Additionally, there are limits to where on the drive the BIOS can boot from. More information can be found in Linux Partition HOWTO, but this section will include a brief overview to help you plan most situations.
``Primary'' partitions are the original partitioning scheme for PC disks. However, there can only be four of them. To get past this limitation, ``extended'' or ``logical'' partitions were invented. By setting one of your primary partitions as an extended partition, you can subdivide all the space allocated to that partition into logical partitions. There is no limitation to the number of logical partitions you can create; however, you can only have one extended partition per drive.
Linux limits the partitions per drive to 15 partitions for SCSI disks (3 usable primary partitions, 12 logical partitions), and 63 partitions on an IDE drive (3 usable primary partitions, 60 logical partitions).
The last issue about PC BIOS which you need to know is that your boot partition, that is, the partition containing your kernel image, needs to be contained within the first 1024 cylinders of the drive. Since usually the root partition is also your boot partition, you need to make sure you root partition fits into the first 1024 cylinders.
If you have a large disk, you might have to use cylinder translations techniques, which you can set in your BIOS, such as LBA translation mode. More information about issues with large disks can be found in the Large Disk HOWTO. If you are using a cylinder translation scheme, then your boot partition has to fit within the translated representation of the 1024th cylinder.
Linux disks and partition names are different than under other operating systems. You need to know the names that Linux uses when you partition. Here's the basic naming scheme:
The partitions on each disk are represented by appending a decimal number to the disk name: ``sda1'' and ``sda2'' represent the first and second partitions of the first SCSI disk drive in your system.
Here is a real-life example. Let's assume you have a system with 2 SCSI disks, one at SCSI address 2 and the other at SCSI address 4. The first disk (at address 2) is then named ``sda'', and the second ``sdb''. If the ``sda'' drive has 5 partitions on it, these will be named ``sda1'', ``sda2'', ..., ``sda5''. The same applies to the ``sdb'' disk and its partitions.
Note that if you have two SCSI host bus adapters (i.e., controllers), the order of the drives can get confusing. The best solution in this case is to watch the boot messages, assuming you know yourself the drive models.
Linux represents the primary partitions as the drive name, plus the
numbers 1 through 4. For example, the first primary partition on the
first IDE drive is /dev/hda1. The logical partitions are
numbered starting at 5, so the first logical partition on that same
drive is /dev/hda5. Remember that the extended
partition, that is, the primary partition holding the logical
partitions, is not usable by itself.
As described above, you should definitely have a separate smaller root
partition, and a larger /usr partition, if you have the
space. For examples, see below. For most users, the two partitions
initially mentioned is sufficient. This is especially appropriate
when you have a single small disk, since breaking out lots of
partitions can waste space.
In some cases, you might need a separate /usr/local partition
if you plan to install many programs that are not part of the
Debian distribution. If your machine will be a mail server, you might
need to make /var/spool/mail a separate partition. Often,
putting /tmp on it's own partition, for instance 20 to 32MB,
is a good idea. If you are setting up a server with lots of user
accounts, it's generally good to have a separate, large
/home partition. In general, the partitioning situation
varies from computer to computer depending on its uses.
For very complex systems, you should see the Multi Disk HOWTO. This contains in-depth information, mostly of interest to ISPs and people setting up servers.
With respect to the issue of swap partition size, there are many views. One rule of thumb which works well is to use as much swap as you have system memory, although there probably isn't much point in going over 64MB of swap for most users. It also shouldn't be smaller than 16MB, in most cases. Of course, there are exceptions to these rules. If you are trying to solve 10000 simultaneous equations on a machine with 256MB of memory, you may need a gigabyte (or more) of swap.
Note that Linux for your architecture
will not use more than 128 megabytes of swap on
a single swap partition. However, you can make multiple swap
partitions by hand and edit /etc/fstab after you've
installed to get more than 128 megabytes of swap. If your swap
requirements are this high, however, you should probably try to spread
the swap across different disks (also called ``spindles''). Or you can
try the more recent Linux kernels (2.2 and higher) where this limitation
was relaxed (be careful, it may require other changes in your system).
As an example, the author's home machine has 32MB of RAM and a 1.7GB
IDE drive on /dev/hda. There is a 500MB DOS partition on
/dev/hda1 (should have made it 200MB as it never gets
used). A 32MB swap partition is used on /dev/hda3 and the
rest (about 1.2GB on /dev/hda2) is the Linux partition.
There are two different times that you can partition: prior to the installation of Debian, or during installation of Debian.
If you are manipulating existing FAT or NTFS partitions, it is recommended that you either use the scheme below or native Windows or DOS tools. Otherwise, it is not really necessary to partition from DOS or Windows; the Linux partitioning tools will generally do a better job.
One of the most common installations is onto a system that already contains DOS (including Windows 3.1), Win-32 (such as Windows 95 and 98 (or NT??)), or OS/2 and it is desired to put Debian onto the same disk without destroying the previous system. As explained in the Background, section 4.1, decreasing the size of an existing partition will almost certainly damage the data on that partition unless certain precautions are taken. The method described here, while not guaranteed to protect your data, works extremely well in practice. As a precaution, you should make a backup.
Before going any further, you should have decided how you will be dividing up the disk. The method in this section will only split a partition into two pieces. One will contain the original OS and the other will be used for Debian. During the installation of Debian, you will be given the opportunity to use Debian portion of the disk as you see fit, i.e., as swap or as a filesystem.
The idea is to move all the data on the partition to the beginning, before changing the partition information, so that nothing will be lost. It is important that you do as little as possible between the data movement and repartitioning to minimize the chance of a file being written near the end of the partition as this will decrease the amount of space you can take from the partition.
The first thing needed is a copy of fips which is
available in the tools/ directory on your nearest Debian
mirror. Unzip the archive and copy the files
RESTORRB.EXE, FIPS.EXE and
ERRORS.TXT to a bootable floppy. A bootable floppy can
be created using the command sys a: under DOS.
fips comes with very good documentation which you may
want to read. You will definitely need to read the documentation if
you use a disk compression driver or a disk manager. Create the disk
and read the documentation before you defragment the disk.
The next thing needed is to move all the data to the beginning of the
partition. defrag, which comes standard with DOS 6.0 and
later can easily do the job. See the fips documentation
for a list of other software that may do the trick. Note that if you
have Windows 95, you must run defrag from there, since
DOS doesn't understand the FAT32 type disk allocation tables sometimes
used in Windows 95 and higher.
After running the defragmenter (which can take a while on a large
disk), reboot with the fips disk you created in the
floppy drive. Simply type a:\fips and follow the directions.
Note that there are many other other partition managers out there, in
case fipsdoesn't do the trick for you.
If you are partitioning for DOS drives, or changing the size of DOS
partitions, using Linux tools, many people experience problems working
with the resulting FAT partitions. For instance, some have reported
slow performance, consistent problems with scandisk, or
other wierd errors in DOS or Windows.
Apparently, whenever you create or resize a partition for DOS use,
it's a good idea to fill the first few sectors with zeros. Do this
prior to running DOS's format command, from Linux:
dd if=/dev/zero of=/dev/hdXX bs=512 count=4