Appendix E. An Introduction to Disk Partitions

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 Linux installation will be as simple and painless as possible.

If you are reasonably comfortable with disk partitions, you could skip ahead to the Section called Making Room For Red Hat Linux, for more information on the process of freeing up disk space to prepare for a Red Hat 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 Disk Basic Concepts

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, we will use a simplified diagram of a disk drive to help explain what is really happening when a disk drive is partitioned. Figure E-1, shows a brand-new, unused disk drive.

Figure E-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 will do. 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…

It is Not What You Write, it is How You Write It

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 unformatted drive.

Figure E-2. Disk Drive with a File System

As Figure E-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 as blocks. [1]

Given that file systems make things like directories and files possible, these tradeoffs are usually seen as a small price to pay.

It is also worth noting that there is no single, universal file system. As Figure E-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 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 E-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 E-4. Disk Drive with Data Written to It

As Figure E-4, shows, 14 of the previously-empty blocks are now holding data. However, by simply looking at this picture, we cannot determine exactly how many files reside on this drive. There may be as few as one or as many as 14 files, 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.

Partitions: Turning One Drive Into Many

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 partition table.


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, we will keep it separate in our diagrams.

Figure E-5. Disk Drive with Partition Table

As Figure E-5, shows, the partition table is divided into four sections. 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" will be 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 three.

Table E-1, contains a listing of some popular (and obscure) partition types, along with their numeric values.

Table E-1. Partition Types

Partition TypeValuePartition TypeValue
Empty00Novell Netware 38665
DOS 12-bit FAT01PIC/IX75
XENIX root02Old MINIX80
XENIX usr03Linux/MINUX81
DOS 16-bit <=32M04Linux swap82
Extended05Linux native83
DOS 16-bit >=3206Linux extended85
OS/2 HPFS07Amoeba93
AIX08Amoeba BBT94
AIX bootable09BSD/386a5
OS/2 Boot Manager0aOpenBSDa6
Win95 FAT32 (LBA)0cBSDI fsb7
Win95 FAT16 (LBA)0eBSDI swapb8
Win95 Extended (LBA)0fSyrinxc7
Venix 8028640CP/Mdb
Novell51DOS accesse1
Microport52DOS R/Oe3
GNU HURD63DOS secondaryf2
Novell Netware 28664BBTff

By this point, you might be wondering how all this additional complexity is normally used. See Figure E-6, for an example.

Figure E-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 E-1, it is adequate for the purposes of this discussion. This is a typical partition layout for most newly purchased computers with a consumer version of Microsoft Windows™ preinstalled.

Partitions within Partitions — An Overview of Extended Partitions

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 E-1, there is an "Extended" partition type. It is this partition type that is at the heart of extended partitions.

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 E-7, shows a disk drive with one primary partition and one extended partition containing two logical partitions (along with some unpartitioned free space).

Figure E-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, in reality, it is probably not a good idea to try to define and use more than 12 logical partitions on a single disk drive.)

Now that we have discussed partitions in general, let us see how to use this knowledge to install Red Hat Linux.

Making Room For Red Hat Linux

There are three 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.


Please keep in mind that the following illustrations are simplified in the interest of clarity and do not reflect the exact partition layout that you will encounter when actually installing Red Hat Linux.

Using Unpartitioned Free Space

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 E-8, shows what this might look like.

Figure E-8. Disk Drive with Unpartitioned Free 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 simply 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 Linux). Most pre-installed operating systems are configured to take up all available space on a disk drive (see the Section called Using Free Space from an Active Partition).

Next, we will discuss a slightly more common situation.

Using Space from an Unused Partition

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 E-9, illustrates such a situation.

Figure E-9. Disk Drive With an Unused Partition

If you find yourself in this situation, you can use the space allocated to the unused partition. You will first need to delete the partition, and then create the appropriate Linux partition(s) in its place. You can either delete the partition using the DOS fdisk command, or you will be given the opportunity to do so during a custom installation.

Using Free Space from an Active Partition

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 choices:

Destructive Repartitioning

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 will need 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 software, you can reinstall any software, restore your data, and continue your Red Hat Linux installation. Figure E-10 shows this being done.

Figure E-10. Disk Drive Being Destructively Repartitioned


As Figure E-10, shows, any data present in the original partition will be lost without proper backup!

Non-Destructive Repartitioning

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. You will have to 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 existing data

  • Resize the existing partition

  • Create new partition(s)

Next we will look at each step in a bit more detail.

Compress existing data

As Figure E-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 E-11. Disk Drive Being Compressed

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.

Resize the existing partition

Figure E-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 E-12. Disk Drive with Partition Resized

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 simply delete the new DOS partition and create the appropriate Linux partition(s).

Create new partition(s)

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 you will need to delete the partition that was created during the resizing process. Figure E-13, shows this being done.

Figure E-13. Disk Drive with Final Partition Configuration


The following information is specific to Intel-based computers only.

As a convenience to Red Hat Linux users, the DOS fips utility is included on the Red Hat Linux/x86 CD 1 in the dosutils directory. This is a freely available program that can resize FAT (File Allocation Table) partitions.


Many people have successfully used fips to resize their hard drive partitions. However, because of the nature of the operations carried out by fips and the wide variety of hardware and software configurations under which it must run, Red Hat cannot guarantee that fips will work properly on your system. Therefore, no installation support is available for fips. Use it at your own risk.

That said, if you decide to repartition your hard drive with fips, it is vital that you do two things:

  • Perform a backup — Make two copies of all the important data on your computer. These copies should be to removable media (such as tape or diskettes), and you should make sure they are readable before proceeding.

  • Read the documentation — Completely read the fips documentation, located in the dosutils/fipsdocs subdirectory on Red Hat Linux/x86 CD 1.

Should you decide to use fips, be aware that after fips runs you will be left with two partitions: the one you resized, and the one fips created out of the newly freed space. If your goal is to use that space to install Red Hat Linux, you should delete the newly created partition, either by using fdisk under your current operating system or while setting up partitions during a custom installation.

Partition Naming Scheme

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 labeled.

  • The drive letter can then be used to refer to that partition as well as the file system contained on that partition.

Red Hat 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 filenames in the form:


Here is how to decipher the partition naming scheme:


This string 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. You will normally see 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 Linux. Of course, this does not mean that Red Hat 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 will make things easier to understand when you are setting up the partitions Red Hat Linux requires.

Disk Partitions and Other Operating Systems

If your Red Hat Linux partitions will be 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. Information on creating disk partitions compatible with other operating systems is available in several HOWTOs and Mini-HOWTOs, available on the Red Hat Linux Documentation CD in the HOWTO and HOWTO/mini directories. In particular, the Mini-HOWTOs whose names start with Linux+ are quite helpful.


If Red Hat Linux/x86 will coexist on your machine with OS/2, you must create your disk partitions with the OS/2 partitioning software —— otherwise, OS/2 may not recognize the disk partitions. During the installation, do not create any new partitions, but do set the proper partition types for your Linux partitions using the Linux fdisk.

Disk Partitions and Mount Points

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 mount point).

For example, if partition /dev/hda5 were mounted on /usr, that would mean that all files and directories under /usr would 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 /dev/hda5.

How Many Partitions?

At this point in the process of preparing to install Red Hat Linux, you will need to 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 the issue.

Keeping this in mind, we recommend that, unless you have a reason for doing otherwise, you should at least create the following partitions:

  • A swap partition — Swap partitions are used to support virtual memory. In other words, data is written to swap when there is not RAM to hold the data your system is processing. You must create a swap partition to correctly use Red Hat Linux. The minimum size of your swap partition should be equal to twice the amount of your computer's RAM or 32 MB, whichever is larger.

  • A /boot partition — The partition mounted on /boot contains the operating system kernel (which allows your system to boot Red Hat Linux), along with a few other files used during the bootstrap process.


    Make sure you read the Section called One Last Wrinkle: Using GRUB or LILO — the information there applies to the /boot partition!

    Due to the limitations of most PC BIOSes, creating a small partition to hold these files is a good idea. For most users, a 32 MB boot partition is sufficient.

  • A root partition (/) — The root partition is where / (the root directory) resides. In this partitioning layout, all files (except those stored in /boot) reside on the root partition. Because of this, it is in your best interest to maximize the size of your root partition. For example, a 1.2 GB root partition may permit the equivalent of a workstation installation (with very little free space), while a 3.4 GB root partition may let you install every package. Obviously, the more space you can give the root partition, the better.

Specific recommendations concerning the proper size for various Red Hat Linux partitions can be found in the Section called Which Installation Class is Best For You? in Chapter 1.

One Last Wrinkle: Using GRUB or LILO

GRUB and LILO are the most commonly used methods to boot Red Hat Linux on Intel-based systems. As operating system loaders, they operate "outside" of any operating system, using only the Basic I/O System (or BIOS) built into the computer hardware itself. This section describes GRUB and LILO's interactions with PC BIOSes and is specific to Intel-compatible computers.

BIOS-Related Limitations Impacting GRUB and LILO

GRUB and LILO are subject to some limitations imposed by the BIOS in most Intel-based computers. Specifically, most BIOSes cannot access more than two hard drives, and they cannot access any data stored beyond cylinder 1023 of any drive. Note that some recent BIOSes do not have these limitations, but this is by no means universal.

All the data GRUB and LILO need to access at boot time (including the Linux kernel) is located in the /boot directory. If you follow the partition layout recommended above or are performing a workstation or server install, the /boot directory will be in a small, separate partition. Otherwise, it may reside in the root partition (/). In either case, the partition in which /boot resides must conform to the following guidelines if you are going to use GRUB or LILO to boot your Red Hat Linux system:

On First Two IDE Drives

If you have 2 IDE (or EIDE) drives, /boot must be located on one of them. Note that this two-drive limit also includes any IDE CD-ROM drives on your primary IDE controller. So, if you have one IDE hard drive, and one IDE CD-ROM on your primary controller, /boot must be located on the first hard drive only, even if you have other hard drives on your secondary IDE controller.

On First IDE or First SCSI Drive

If you have one IDE (or EIDE) drive and one or more SCSI drives, /boot must be located either on the IDE drive or the SCSI drive at ID 0. No other SCSI IDs will work.

On First Two SCSI Drives

If you have only SCSI hard drives, /boot must be located on a drive at ID 0 or ID 1. No other SCSI IDs will work.

Partition Completely Below Cylinder 1023

No matter which of the above configurations apply, the partition that holds /boot must be located entirely below cylinder 1023. If the partition holding /boot straddles cylinder 1023, you may face a situation where GRUB and LILO will work initially (because all the necessary information is below cylinder 1023) but will fail if a new kernel is to be loaded and that kernel resides above cylinder 1023.

As mentioned earlier, it is possible that some of the newer BIOSes may permit GRUB and LILO to work with configurations that do not meet these guidelines. Likewise, some of GRUB and LILO's more esoteric features may be used to get a Linux system started, even if the configuration does not meet our guidelines. However, due to the number of variables involved, Red Hat cannot support such efforts.


Disk Druid, as well as the workstation and server installations, takes these BIOS-related limitations into account.



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.