Red Hat Linux 7.2: The Official Red Hat Linux Alpha Installation Guide
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# 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 preinstalled 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 filesystem") writes information to the drive, creating order out of the empty space in an unformatted drive.

Figure E-2. Disk Drive with a Filesystem

As Figure E-2 implies, the order imposed by a filesystem involves some trade-offs:

- A small percentage of the drive's available space is used to store filesystem-related data and can be considered as overhead.

- A filesystem splits the remaining space into small, consistently-sized segments. For Linux, these segments are known as blocks. [1]

Given that filesystems 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 filesystem. As Figure E-3 shows, a disk drive may have one of many different filesystems written on it. As you might guess, different filesystems tend to be incompatible; that is, an operating system that supports one filesystem (or a handful of related filesystem 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 filesystems (including many commonly used by other operating systems), making data interchange between different filesystems easy.

Figure E-3. Disk Drive with a Different Filesystem

Of course, writing a filesystem 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 filesystems were never designed to support anything above a certain capacity. Or the filesystems could support larger drives with a greater capacity, but the overhead imposed by the filesystem 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.

| Note
| 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 filesystem 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 filesystem 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 Type | Value | Partition Type | Value
--- | --- | --- | ---
Empty | 00 | Novell Netware 386 | 65
DOS 12-bit FAT | 01 | PIC/IX | 75
XENIX root | 02 | Old MINIX | 80
XENIX usr | 03 | Linux/MINUX | 81
DOS 16-bit <=32M | 04 | Linux swap | 82
Extended | 05 | Linux native | 83
DOS 16-bit >=32 | 06 | Linux extended | 85
OS/2 HPFS | 07 | Amoeba | 93
AIX | 08 | Amoeba BBT | 94
AIX bootable | 09 | BSD/386 | a5
OS/2 Boot Manager | 0a | OpenBSD | a6
Win95 FAT32 | 0b | NEXTSTEP | a7
Win95 FAT32 (LBA) | 0c | BSDI fs | b7
Win95 FAT16 (LBA) | 0e | BSDI swap | b8
Win95 Extended (LBA) | 0f | Syrinx | c7
Venix 80286 | 40 | CP/M | db
Novell | 51 | DOS access | e1
Microport | 52 | DOS R/O | e3
GNU HURD | 63 | DOS secondary | f2
Novell Netware 286 | 64 | BBT | ff

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's look at each scenario in order.

| Note
| 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).

Let's move on to 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.

  | Caution
  | 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

  | Caution
  | 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)

Let's take a 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

## 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 filesystem 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:

    /dev/xxyN

Here is how to decipher the partition naming scheme:

`/dev/`

  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/`.

``xx``

  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).

``y``

  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).

``N``

  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.

| Note
| 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.

## 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.

- 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. A 1.2 GB root partition will permit the equivalent of a workstation installation (with very little free space), while a 3.4 GB root partition will 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.

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