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An Introduction to Disk Partitions

This appendix is not necessarily applicable to architectures other than AMD64 and Intel 64. However, the general concepts mentioned here may apply.

This section discusses basic disk concepts, disk repartitioning strategies, the partition naming scheme used by Linux systems, and related topics.

If you are comfortable with disk partitions, you can skip ahead to Strategies for Disk Repartitioning for more information on the process of freeing up disk space to prepare for a Fedora installation.

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 have a understanding of the underlying hardware; however, since the theory is very complicated and expansive, only the basic concepts will be explained here. This appendix uses a set of simplified diagrams of a disk drive to help explain what is the process and theory behind partitions.

An Unused Disk Drive, shows a brand-new, unused disk drive.

Figura 1. An Unused Disk Drive

File Systems

To store data on a disk drive, it is necessary to format the disk drive first. Formatting (usually known as "making a file system") writes information to the drive, creating order out of the empty space in an unformatted drive.

Imagen de una unidad de disco formateado.

Figura 2. Unidad de Disco con un Sistema de Archivos

As Disk Drive with a File System, implies, the order imposed by a file system involves some trade-offs:

Un pequeño porcentaje del espacio disponible del controlador se usa para almacenar datos relacionados con el sistema de archivos y puede ser considerado como sobrecarga.
A file system splits the remaining space into small, consistently-sized segments. For Linux, these segments are known as blocks.

^[1]

Note that there is no single, universal file system. As Disk Drive with a Different File System, shows, a disk drive may have one of many different file systems written on it. 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. However, Fedora supports a wide variety of file systems (including many commonly used by other operating systems such as Microsoft Windows), making data interchange between different file systems easy.

Imagen de una unidad de disco con un sistema de archivos diferente.

Figura 3. Unidad de Disco con un Sistema de Archivos Diferente

Escribir un sistema de archivos en un disco es solo el primer paso. El objetivo de este proceso es realmente almacenar y recuperar datos. La figura de abajo muestra una unidad de disco después de que se hayan escrito en él algunos datos:

Imagen de una unidad de disco con datos escritos en él.

Figura 4. Unidad de Disco con Datos Escritos en él

As Disk Drive with Data Written to It, 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.

Como con la mayoría de las tecnologías relacionadas con los ordenadores, las unidades de disco han cambiado desde su introducción. En particular, han llegado a ser más grandes. No mayores en tamaño físico, sino más grandes en su capacidad de almacenar información. Y esta capacidad adicional ha conducido a un cambio fundamental en la manera en la que los unidades de disco se utilizan.

Particiones: Volviendo un Dispositivo en Muchos

Los dispositivos de disco pueden ser divididos en particiones. A cada partición se puede acceder como si fuera un disco separado. Esto se hace a través de la creación de una tabla de partición.

Hay diversas razones para asignar espacio de disco en particiones de disco separadas, por ejemplo:

La separación lógica de los datos del sistema operativo de los datos del usuario
La capacidad de usar diferentes sistemas de archivos
La capacidad de correr múltiples sistemas operativos en una máquina

There are currently two partitioning layout standards for physical hard disks: Master Boot Record (MBR) and GUID Partition Table (GPT). MBR is an older method of disk partitioning used with BIOS-based computers. GPT is a newer partitioning layout that is a part of the Unified Extensible Firmware Interface (UEFI). This section and Partitions Within Partitions - An Overview of Extended Partitions mainly describe the Master Boot Record (MBR) disk partitioning scheme. For information about the GUID Partition Table (GPT) partitioning layout, see GUID Partition Table (GPT).

Si bien los diagramas de este capítulo muestran la tabla de particiones separada del dispositivo de disco real esto no es totalmente cierto. En realidad la tabla de partición está almacenada muy al principio del disco, antes de cualquier sistema de archivo o datos de usuario. Pero por claridad están separadas en nuestros diagramas.

Imagen de un dispositivo de disco no usado con una tabla de particiones.

Figura 5. Dispositivo de Disco con Tabla de Particiones

As Disk Drive with Partition Table 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.

Cada entrada de la tabla de particiones contiene diversas características importantes de la partición:

Los puntos del disco donde la partición empieza y termina
Whether the partition is "active"
El tipo de partición

The starting and ending points 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.

El tipo es un número que identifica el uso anticipado de la partición. Algunos sistemas operativo usan el tipo de partición para denotar un tipo específico de sistema de archivos, para marcar la partición como asociada con un sistema operativo concreto, para indicar que la partición contiene un sistema de archivo arrancable o alguna combinación de las tres.

See Disk Drive With Single Partition for an example of a disk drive with single partition.

Imagen de dispositivo de disco con única partición.

Figura 6. Dispositivo de Disco Con Única Partición

La única partición de este ejemplo está etiquetada como DOS. Esta etiqueta muestra el tipo de partición, con DOS siendo una de las más comunes. La tabla de abajo muestra algunos de los tipos de partición más comúnmente usados y los números hexadecimales usados para representarlas.

Table 1. Tipos de Partición
Tipo de Partición	Valor	Tipo de Partición	Valor
Vacío	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	Intercambio Linux	82
Extendida	05	Linux nativo	83
DOS 16-bit >=32	06	Linux extendida	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
PReP Boot	41	DOS R/O	e3
GNU HURD	63	DOS secondary	f2
Novell Netware 286	64	BBT	ff

Particiones Dentro de Particiones - Una Descripción General de las Particiones Extendidas

In case four partitions are insufficient for your needs, you can use extended partitions to create up additional partitions. You do this by setting the type of a partition to "Extended".

An extended partition is like a disk drive in its own right - it has its own partition table which points to one or more partitions (now called logical partitions, as opposed to the four primary partitions) contained entirely within the extended partition itself. Disk Drive With Extended Partition, shows a disk drive with one primary partition and one extended partition containing two logical partitions (along with some unpartitioned free space).

Imagen de un dispositivo de disco con una partición extendida.

Figura 7. Dispositivo de Disco Con Partición Extendida

Como implica esta figura, hay una diferencia entre las particiones primarias y lógicas - solo puede haber cuatro particiones primarias pero no hay un número fijo de particiones lógicas que pueden existir. Sin embargo, debido a la manera en la que se accede a dichas particiones en Linux, no se deberían definir más de 12 particiones lógicas en un dispositivo de disco.

Tabla de Partición GUID (GPT)

La Tabla de Partición GUID (GPT) es un nuevo esquema de particionado basado en el uso de los Identificadores Únicos Globales (GUID). GPT fue desarrollado para afrontar las limitaciones de la tabla de partición MBR especialmente con el limitado espacio de almacenamiento máximo direccionable de un disco. A diferencia de MBR, que es incapaz de direccionar almacenamiento mayor de 2.2 terabytes, GPT puede usado con discos más grandes que eso; el tamaño de disco máximo direccionable es de 2.2 zettabytes. Además, GPT de modo predeterminado soporta la creación de hasta 128 particiones primarias. Este número podría ser extendido asignando más espacio a la tabla de partición.

Lo discos GPT utilizan direccionamiento de bloque lógico (LBA) y el diseño de partición es como sigue:

To preserve backward compatibility with MBR disks, the first sector (LBA 0) of GPT is reserved for MBR data and it is called "protective MBR".
La cabecera principal GPT* empieza en el segundo bloque lógico del dispositivo (*LBA 1). La cabecera contiene la GUID del disco, la localización de la tabla de partición primaria, la localización de la cabecera GPT secundaria y la suma de comprobación CRC32 de ella misma y de la tabla de partición primaria. También especifica el número de entradas de partición de la tabla.
La tabla primaria *GPT* incluye, de forma predeterminada, 128 entradas de partición, cada una con un tamaño de entrada de 128 bytes, su tipo de partición GUID y una partición única GUID.
La tabla GPT secundaria es idéntica a la tabla GPT primaria. Se usa principalmente como copia de seguridad de la tabla para su recuperación en el caso de corrupción de la tabla de partición primaria.
La cabecera GPT secundaria se localiza en el último sector lógico del disco y puede ser usada para recuperar información GPT en el caso de corrupción de la cabecera primaria. Contiene la GUID del disco, la localización de la tabla de partición secundaria y de la cabecera GPT primaria, la suma de comprobación CRC32 y la tabla de partición de secundaria y el número de posibles entradas de partición.

Debe haber una partición de arranque BIOS para que el cargador de arranque se instale correctamente en un disco que contenga una GPT (Tabla de Partición GUID). Esto incluye los discos inicializados por Anaconda. Si el disco ya contiene una partición de arranque BIOS puede ser reutilizada.

Estrategias para el Reparticionado de Disco

There are several different ways that a disk can be repartitioned. This section discusses the following possible approaches:

Unpartitioned free space is available
An unused partition is available
Free space in an actively used partition is available

Note that this section discusses the aforementioned concepts only theoretically and it does not include any procedures showing how to perform disk repartitioning step-by-step. Such detailed information are beyond the scope of this document.

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 Fedora.

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. Disk Drive with Unpartitioned Free Space, shows what this might look like.

Image of a disk drive with unpartitioned free space

Figura 8. Disk Drive with Unpartitioned Free Space

In the above example, 1 represents an undefined partition with unallocated space and 2 represents a defined partition with allocated space.

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 Fedora). Most pre-installed operating systems are configured to take up all available space on a disk drive (see Using Free Space from an Active Partition).

Using Space from an Unused Partition

In this case, maybe you have one or more partitions that you do not use any longer. Disk Drive with an Unused Partition, illustrates such a situation.

Image of a disk drive with an unused partition

Figura 9. Disk Drive with an Unused Partition

In the above example, 1 represents an unused partition and 2 represents reallocating an unused partition for Linux.

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.

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

In this case, the single large partition is deleted and several smaller ones are created instead. Any data held in the original partition is destroyed. This means that making a complete backup is necessary. It is highly recommended to make two backups, use verification (if available in your backup software), and try to read data from the backup before deleting the partition.

If an operating system was installed on that partition, it must be reinstalled if you want to use that system as well. Be aware that some computers sold with pre-installed operating systems may not include the installation media to reinstall the original operating system. You should check whether 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 software, restore your data, and start the installation. Disk Drive Being Destructively Repartitioned shows this being done.

Image of a disk drive being destructively repartitioned

Figura 10. Disk Drive Being Destructively Repartitioned

In the above example, 1 represents before and 2 represents after.

Any data previously present in the original partition is lost.

Non-Destructive Repartitioning

With non-destructive repartitioning you execute a program that makes a big partition smaller without losing any of the files stored in that partition. This method is usually reliable, but can be very time-consuming on large drives.

While the process of non-destructive repartitioning is rather straightforward, there are three steps involved:

Compress and backup existing data
Resize the existing partition
Create new partition(s)

Each step is described further in more detail.

Compress Existing Data

As the following figure 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.

Figura 11. Disk Drive Being Compressed

In the above example, 1 represents before and 2 represents after.

This step is crucial. Without it, the location of the 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

Disk Drive with Partition Resized 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.

Image of a disk drive with a resized partition

Figura 12. Disk Drive with Partition Resized

In the above example, 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 illustrated here, it would be best to 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 that you must delete the partition that was created during the resizing process. Disk Drive with Final Partition Configuration, shows this being done.

Image of a disk drive with final partition configuration

Figura 13. Disk Drive with Final Partition Configuration

In the above example, 1 represents before and 2 represents after.

Partition Naming Schemes and Mount Points

A common source of confusion for users unfamiliar with Linux 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. This section describes the main principles of partition naming scheme and the way how partitions are accessed in Fedora.

Partition Naming Scheme

Linux uses a naming scheme that is file-based, with file names in the form of /dev/xxyN.

Device and partition names consist of the following:

/dev/: This is the name of the directory in which all device files reside. Because 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, usually sd.
y: This letter indicates which device the partition is on. For example, /dev/sda for the first hard disk, /dev/sdb for the second, and so on.
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/sda3 is the third primary or extended partition on the first hard disk, and /dev/sdb6 is the second logical partition on the second hard disk.

Even if your system can identify and refer to all types of disk partitions, it might not be able to read the file system and therefore access stored data on every partition type. However, in many cases, it is possible to successfully access data on a partition dedicated to another operating system.

Disk Partitions and Mount Points

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/sda5 is mounted on /usr/, that would mean that all files and directories under /usr/ physically reside on /dev/sda5. So the file /usr/share/doc/FAQ/txt/Linux-FAQ would be stored on /dev/sda5, while the file /etc/gdm/custom.conf would not.

Continuing the 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/sda7) could be mounted on /usr/local/, meaning that /usr/local/man/whatis would then reside on /dev/sda7 rather than /dev/sda5.

How Many Partitions?

At this point in the process of preparing to install Fedora, you should give some consideration to the number and size of the partitions to be used by your new operating system. However, there is no one right answer to this question. It depends on your needs and requirements.

Unless you have a reason for doing otherwise, you should at least create a /boot partition and a / (root) partition. Depending on your system’s hardware specifications, additional partitions may be necessary, such as /boot/efi for 64-bit AMD and Intel systems with UEFI firmware, a biosboot partition for AMD and Intel systems with a GUID Partition Table (GPT) label on the system disk, or a PReP Boot partition on IBM Power Systems servers.

See Recommended Partitioning Scheme for more information about the recommended partitioning scheme.

1. Blocks really are consistently sized, unlike our illustrations. Keep in mind, also, that an average disk drive contains thousands of blocks. The picture is simplified for the purposes of this discussion.

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