NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
Red Hat Enterprise Linux 4: Installation Guide for x86, Itanium™, AMD64, and Intel® Extended Memory 64 Technology (Intel® EM64T)
This appendix is not necessarily applicable to non-x86-based
architectures. However, the general concepts mentioned here may apply.
Disk partitions are a standard part of the personal computer landscape
and have been for quite some time. However, with many people purchasing
computers featuring pre-installed operating systems, relatively few people
understand how partitions work. This chapter attempts to explain the
reasons for and use of disk partitions so your Red Hat Enterprise Linux
installation is as simple and painless as possible.
If you are reasonably comfortable with disk partitions, you could skip
ahead to Section D.1.4 Making Room For Red Hat Enterprise Linux, for more information on
the process of freeing up disk space to prepare for a Red Hat Enterprise Linux
installation. This section also discusses the partition naming
scheme used by Linux systems, sharing disk space with other operating
systems, and related topics.
Hard disks perform a very simple function — they store data
and reliably retrieve it on command.
When discussing issues such as disk partitioning, it is important to
know a bit about the underlying hardware. Unfortunately, it is easy to
become bogged down in details. Therefore, this appendix uses a simplified
diagram of a disk drive to help explain what is really happening when a
disk drive is partitioned. Figure D-1,
shows a brand-new, unused disk drive.
Figure D-1. An Unused Disk Drive
Not much to look at, is it? But if we are talking about disk drives
on a basic level, it is adequate. Say that we would like to store some
data on this drive. As things stand now, it will not work. There is
something we need to do first…
Experienced computer users probably got this one on the first try.
We need to format the drive. Formatting
(usually known as "making a file system") writes
information to the drive, creating order out of the empty space in an
Figure D-2. Disk Drive with a File System
As Figure D-2, implies, the
order imposed by a file system involves some trade-offs:
A small percentage of the drive's available space is used to
store file system-related data and can be considered as overhead.
A file system splits the remaining space into small,
consistently-sized segments. For Linux, these segments are known
Given that file systems make things like directories and files
possible, these trade-offs are usually seen as a small price to pay.
It is also worth noting that there is no single, universal
file system. As Figure D-3, shows,
a disk drive may have one of many different file systems written on
it. As you might guess, different file systems tend to be incompatible;
that is, an operating system that supports one file system (or a
handful of related file system types) may not support another. This
last statement is not a hard-and-fast rule, however. For example,
Red Hat Enterprise Linux supports a wide variety of file systems
(including many commonly used by other operating systems), making data
interchange between different file systems easy.
Figure D-3. Disk Drive with a Different File System
Of course, writing a file system to disk is only the beginning.
The goal of this process is to actually store and
retrieve data. Let us take a look at our drive
after some files have been written to it.
Figure D-4. Disk Drive with Data Written to It
As Figure D-4, shows, some
of the previously-empty blocks are now holding data. However, by just
looking at this picture, we cannot determine exactly how many files
reside on this drive. There may only be one file or many, as all files
use at least one block and some files use multiple blocks. Another
important point to note is that the used blocks do not have to form a
contiguous region; used and unused blocks may be interspersed. This
is known as fragmentation. Fragmentation can
play a part when attempting to resize an existing partition.
As with most computer-related technologies, disk drives changed
over time after their introduction. In particular, they got
bigger. Not larger in physical size, but bigger in their capacity to
store information. And, this additional capacity drove a fundamental
change in the way disk drives were used.
As disk drive capacities soared, some people began to wonder if
having all of that formatted space in one big chunk was such a great
idea. This line of thinking was driven by several issues, some
philosophical, some technical. On the philosophical side, above a
certain size, it seemed that the additional space provided by a larger
drive created more clutter. On the technical side, some file systems
were never designed to support anything above a certain capacity. Or
the file systems could support larger drives with
a greater capacity, but the overhead imposed by the file system to
track files became excessive.
The solution to this problem was to divide disks into
partitions. Each partition can be accessed as
if it was a separate disk. This is done through the addition of a
While the diagrams in this chapter show the
partition table as being separate from the actual disk drive, this is
not entirely accurate. In reality, the partition table is stored at
the very start of the disk, before any file system or user data. But
for clarity, they are separate in our diagrams.
Figure D-5. Disk Drive with Partition Table
As Figure D-5 shows, the
partition table is divided into four sections or four
primary partitions. A primary partition is a
partition on a hard drive that can contain only one logical drive (or
section). Each section can hold the information necessary to define a
single partition, meaning that the partition table can define no more
than four partitions.
Each partition table entry contains several important
characteristics of the partition:
The points on the disk where the partition starts and ends
Whether the partition is "active"
The partition's type
Let us take a closer look at each of these characteristics. The
starting and ending points actually define the partition's size and
location on the disk. The "active" flag is used by some operating
systems' boot loaders. In other words, the operating system in the
partition that is marked "active" is booted.
The partition's type can be a bit confusing. The type is a number that
identifies the partition's anticipated usage. If that statement sounds
a bit vague, that is because the meaning of the partition type is a bit
vague. Some operating systems use the partition type to denote a
specific file system type, to flag the partition as being associated
with a particular operating system, to indicate that the partition
contains a bootable operating system, or some combination of the
By this point, you might be wondering how all this additional
complexity is normally used. Refer to Figure D-6, for an example.
Figure D-6. Disk Drive With Single Partition
In many cases, there is only a single partition spanning the
entire disk, essentially duplicating the method used before
partitions. The partition table has only one entry used, and it points
to the start of the partition.
We have labeled this partition as being of the "DOS"
type. Although it is only one of several possible partition types
listed in Table D-1, it is adequate for the
purposes of this discussion.
Table D-1, contains a listing of some
popular (and obscure) partition types, along with their hexadecimal
Of course, over time it became obvious that four partitions would
not be enough. As disk drives continued to grow, it became more and
more likely that a person could configure four reasonably-sized
partitions and still have disk space left over. There needed to be
some way of creating more partitions.
Enter the extended partition. As you may have noticed in Table D-1, there is an "Extended" partition
type. It is this partition type that is at the heart of extended
When a partition is created and its type is set to "Extended," an
extended partition table is created. In essence, the extended
partition is like a disk drive in its own right — it has a
partition table that points to one or more partitions (now called
logical partitions, as opposed to the four
primary partitions) contained entirely within
the extended partition itself. Figure D-7, shows a disk drive with one
primary partition and one extended partition containing two logical
partitions (along with some unpartitioned free space).
Figure D-7. Disk Drive With Extended Partition
As this figure implies, there is a difference between primary and
logical partitions — there can only be four primary partitions,
but there is no fixed limit to the number of logical partitions that
can exist. However, due to the way in which partitions are accessed in
Linux, you should avoid defining more than 12 logical partitions on a
single disk drive.
Now that we have discussed partitions in general, let us review how to use
this knowledge to install Red Hat Enterprise Linux.
The following list presents some possible scenarios you may face
when attempting to repartition your hard disk:
Unpartitioned free space is available
An unused partition is available
Free space in an actively used partition is available
Let us look at each scenario in order.
Keep in mind that the following
illustrations are simplified in the interest of clarity and do not
reflect the exact partition layout that you encounter when actually
installing Red Hat Enterprise Linux.
In this situation, the partitions already defined do not span
the entire hard disk, leaving unallocated space that is not part of
any defined partition. Figure D-8,
shows what this might look like.
Figure D-8. Disk Drive with Unpartitioned Free Space
In Figure D-8,
1 represents an undefined partition with
unallocated space and 2 represents a
defined partition with allocated space.
If you think about it, an unused hard disk also falls into this
category. The only difference is that all the
space is not part of any defined partition.
In any case, you can create the necessary partitions from the
unused space. Unfortunately, this scenario, although very simple, is
not very likely (unless you have just purchased a new disk just for
Red Hat Enterprise Linux). Most pre-installed operating systems are configured to take
up all available space on a disk drive (refer to Section D.1.4.3 Using Free Space from an Active Partition).
Next, we will discuss a slightly more common situation.
In this case, maybe you have one or more partitions that you do
not use any longer. Perhaps you have dabbled with another operating
system in the past, and the partition(s) you dedicated to it never
seem to be used anymore. Figure D-9, illustrates such a
Figure D-9. Disk Drive With an Unused Partition
In Figure D-9,
1 represents an unused partition and
2 represents reallocating an unused
partition for Linux.
If you find yourself in this situation, you can use the space
allocated to the unused partition. You first must delete the
partition and then create the appropriate Linux partition(s) in its
place. You can delete the unused partition and manually create new
partitions during the installation process.
This is the most common situation. It is also, unfortunately,
the hardest to handle. The main problem is that, even if you have
enough free space, it is presently allocated to a partition that is
already in use. If you purchased a computer with pre-installed
software, the hard disk most likely has one massive partition
holding the operating system and data.
Aside from adding a new hard drive to your system, you have two
Basically, you delete the single large partition and
create several smaller ones. As you might imagine, any data
you had in the original partition is destroyed. This means
that making a complete backup is necessary. For your own
sake, make two backups, use verification (if available in your
backup software), and try to read data from your backup
before you delete the partition.
If there was an operating system
of some type installed on that partition, it needs to be
reinstalled as well. Be aware that some computers sold with
pre-installed operating systems may not include the CD-ROM
media to reinstall the original operating system. The best
time to notice if this applies to your system is
before you destroy your original
partition and its operating system installation.
After creating a smaller partition for your existing
operating system, you can reinstall any software, restore your
data, and start your Red Hat Enterprise Linux installation. Figure D-10 shows this being
Figure D-10. Disk Drive Being Destructively Repartitioned
In Figure D-10,
1 represents before and
2 represents after.
As Figure D-10, shows, any data
present in the original partition is lost without proper
Here, you run a program that does the seemingly
impossible: it makes a big partition smaller without losing
any of the files stored in that partition. Many people have
found this method to be reliable and trouble-free. What
software should you use to perform this feat? There are
several disk management software products on the market. Do
some research to find the one that is best for your situation.
While the process of non-destructive repartitioning is
rather straightforward, there are a number of steps involved:
Compress and backup existing data
Resize the existing partition
Create new partition(s)
Next we will look at each step in a bit more detail.
As Figure D-11, shows, the
first step is to compress the data in your existing partition.
The reason for doing this is to rearrange the data such that it
maximizes the available free space at the "end" of the partition.
Figure D-11. Disk Drive Being Compressed
In Figure D-11,
1 represents before and
2 represents after.
This step is crucial. Without it, the location of your data
could prevent the partition from being resized to the extent
desired. Note also that, for one reason or another, some data
cannot be moved. If this is the case (and it severely restricts
the size of your new partition(s)), you may be forced to
destructively repartition your disk.
Figure D-12, shows the
actual resizing process. While the actual result of the resizing
operation varies depending on the software used, in most cases the
newly freed space is used to create an unformatted partition of
the same type as the original partition.
Figure D-12. Disk Drive with Partition Resized
In Figure D-12,
1 represents before and
2 represents after.
It is important to understand what the resizing software you
use does with the newly freed space, so that you can take the
appropriate steps. In the case we have illustrated, it would be
best to delete the new DOS partition and create the appropriate
As the previous step implied, it may or may not be necessary
to create new partitions. However, unless your resizing software
is Linux-aware, it is likely that you must delete the partition
that was created during the resizing process. Figure D-13, shows this being done.
Figure D-13. Disk Drive with Final Partition Configuration
In Figure D-13,
1 represents before and
2 represents after.
The following information is specific to
x86-based computers only.
As a convenience to our customers, we provide the
parted utility. This is a freely available program
that can resize partitions.
If you decide to repartition your hard drive with
parted, it is important that you be familiar with
disk storage and that you perform a backup of your computer
data. You should make two copies of all the important data on your
computer. These copies should be to removable media (such as tape,
CD-ROM, or diskettes), and you should make sure they are readable
Should you decide to use parted, be aware
that after parted runs you are left with
two partitions: the one you resized, and the
one parted created out of the newly freed space.
If your goal is to use that space to install Red Hat Enterprise Linux, you should
delete the newly created partition, either by using the partitioning
utility under your current operating system or while setting up
partitions during installation.
Linux refers to disk partitions using a combination of letters and
numbers which may be confusing, particularly if you are used to the "C
drive" way of referring to hard disks and their partitions. In the
DOS/Windows world, partitions are named using the following method:
Each partition's type is checked to determine if it can be
read by DOS/Windows.
If the partition's type is compatible, it is assigned a "drive
letter." The drive letters start with a "C" and move on to the
following letters, depending on the number of partitions to be
The drive letter can then be used to refer to that partition
as well as the file system contained on that partition.
Red Hat Enterprise Linux uses a naming scheme that is more flexible and conveys more
information than the approach used by other operating systems. The
naming scheme is file-based, with file names in the form of
Here is how to decipher the partition naming scheme:
This is the name of the directory in which all device
files reside. Since partitions reside on hard disks, and hard
disks are devices, the files representing all possible
partitions reside in /dev/.
The first two letters of the partition name indicate the
type of device on which the partition resides, usually either
hd (for IDE disks) or
sd (for SCSI disks).
This letter indicates which device the partition is on. For
example, /dev/hda (the first IDE hard disk)
or /dev/sdb (the second SCSI disk).
The final number denotes the partition. The first four
(primary or extended) partitions are numbered
1 through 4. Logical
partitions start at 5. So, for example,
/dev/hda3 is the third primary or extended
partition on the first IDE hard disk, and
/dev/sdb6 is the second logical partition
on the second SCSI hard disk.
There is no part of this naming convention
that is based on partition type; unlike DOS/Windows,
all partitions can be identified under Red Hat Enterprise Linux.
Of course, this does not mean that Red Hat Enterprise Linux
can access data on every type of
partition, but in many cases it is possible to access data on a
partition dedicated to another operating system.
Keep this information in mind; it makes things easier to
understand when you are setting up the partitions Red Hat Enterprise Linux
If your Red Hat Enterprise Linux partitions are sharing a hard disk with
partitions used by other operating systems, most of the time you will
have no problems. However, there are certain combinations of Linux
and other operating systems that require extra care.
One area that many people new to Linux find confusing is the
matter of how partitions are used and accessed by the Linux operating
system. In DOS/Windows, it is relatively simple: Each partition gets
a "drive letter." You then use the correct drive letter to refer to
files and directories on its corresponding partition.
This is entirely different from how Linux deals with partitions
and, for that matter, with disk storage in general. The main
difference is that each partition is used to form part of the storage
necessary to support a single set of files and directories. This is
done by associating a partition with a directory through a process
known as mounting. Mounting a partition makes
its storage available starting at the specified directory (known as a
For example, if partition /dev/hda5 is
mounted on /usr/, that would mean that all files
and directories under /usr/ physically reside on
/dev/hda5. So the file
/usr/share/doc/FAQ/txt/Linux-FAQ would be stored
on /dev/hda5, while the file
/etc/X11/gdm/Sessions/Gnome would not.
Continuing our example, it is also possible that one or more
directories below /usr/ would be mount points for
other partitions. For instance, a partition (say,
/dev/hda7) could be mounted on
/usr/local/, meaning that
/usr/local/man/whatis would then reside on
/dev/hda7 rather than
At this point in the process of preparing to install Red Hat Enterprise Linux, you
must give some consideration to the number and size of the partitions
to be used by your new operating system. The question of "how many
partitions" continues to spark debate within the Linux community and,
without any end to the debate in sight, it is safe to say that there
are probably as many partition layouts as there are people debating
Keeping this in mind, we recommend that, unless you have a reason
for doing otherwise, you should at least create the following
partitions: swap, /boot/
/boot/efi/ partition for Itanium systems),
/var/ partition for Itanium systems, and
Blocks really are consistently sized,
unlike our illustrations. Keep in mind, also, that an average
disk drive contains thousands of blocks. But for the purposes
of this discussion, please ignore these minor discrepancies.