An Introduction to Disk Partitions

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.

As the previous diagram implies, the order imposed by a file system involves some trade-offs:

Note that there is no single, universal file system. As the following diagram shows, a disk drive can 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) might not support another. However, for example, CentOS supports a wide variety of file systems (including many commonly used by other operating systems), making data interchange between different file systems easy.

Writing a file system to disk is only the first step. The goal of this process is to actually store and retrieve data. The figure below shows a drive disk after some data have been written to it:

As the previous diagram 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 might 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 can 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.

Disk drives can be divided into partitions. Each partition can be accessed as if it was a separate disk. This is done through the addition of a partition table.

There are several reasons for allocating disk space into separate disk partitions, for example:

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).

As the previous diagram 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 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.

The type is a number that identifies the partition’s anticipated usage. 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.

The following shows an example of a disk drive with single partition:

The single partition in this example is labeled as DOS. This label shows the partition type, with DOS being one of the most common ones. The table below shows a list of some of the commonly used partition types and hexadecimal numbers used to represent them.

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. The following diagram shows a disk drive with one primary partition and one extended partition containing two logical partitions (along with some unpartitioned free space):

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, no more than 12 logical partitions should be defined on a single disk drive.

GUID Partition Table (GPT) is a newer partitioning scheme based on using Globally Unique Identifiers (GUID). GPT was developed to cope with limitations of the MBR partition table, especially with the limited maximum addressable storage space of a disk. Unlike MBR, which is unable to address storage space larger than 2 TiB (equivalent to approximately 2.2 TB), GPT can be used with hard disks larger than this; the maximum addressable disk size is 2.2 ZiB. In addition GPT, by default, supports creating up to 128 primary partitions. This number could be extended by allocating more space to the partition table.

GPT disks use logical block addressing (LBA) and the partition layout is as follows:

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. The following diagram shows what this might look like:

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 CentOS). 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).

In this case, maybe you have one or more partitions that you do not use any longer. The following diagram illustrates such a situation.

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.

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:

Each step is described further in more detail.

CentOS 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:

In CentOS 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.

At this point in the process of preparing to install CentOS, you must 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.

Keeping this in mind, unless you have a reason for doing otherwise, you should at least create the following partitions: swap, /boot/, and / (root).

For more information, see Recommended Partitioning Scheme (x86) for AMD64, Intel 64, and ARM systems, and Recommended Partitioning Scheme (ppc) for IBM Power Systems servers.