A hard disk in Linux can be subdivided into partitions, which function as individual block devices. You may recall examples like /dev/sda1 and /dev/sda2. Here, /dev/sda represents the entire disk, while /dev/sda1 is the first partition on that disk. Partitions are incredibly useful for separating data. If you need a specific filesystem for a portion of your storage, you can create a new partition for it instead of formatting the entire disk.
The Partition Table
So, what component of a disk tells the OS how the disk is partitioned? The answer is the partition table. This crucial component contains information on how hard drive partitions are organized. The partition table specifies where each partition begins and ends, which partitions are bootable, and what sectors of the disk are allocated to each partition. There are two primary partition table schemes: Master Boot Record (MBR) and GUID Partition Table (GPT).
Understanding Linux Partitions
Disks are composed of partitions that help us organize our data. You can have multiple partitions on a single disk, but they cannot overlap. Any space on the disk not allocated to a partition is known as free space. The types of Linux partitions available depend on the partition table scheme you use. Inside a partition, you can create a filesystem or dedicate it to other purposes, such as swap space.
MBR Partitions
The Master Boot Record (MBR) is the traditional partition table standard.
- It supports primary, extended, and logical partitions.
- MBR has a limit of four primary partitions.
- To create more partitions, one primary partition must be designated as an extended partition (only one is allowed per disk). Within this extended partition, you can create multiple logical partitions, which function like any other partition.
- It supports disks up to 2 terabytes in size.
GPT Partitions
The GUID Partition Table (GPT) is the modern standard for disk partitioning.
- It has only one type of partition, and you can create a large number of them.
- Each partition is assigned a Globally Unique Identifier (GUID).
- GPT is commonly used with UEFI-based booting systems.
Filesystem Structure
As we learned previously, a filesystem is an organized collection of files and directories. At its core, it consists of a database to manage files and the files themselves. Let's explore its structure in more detail.
- Boot block: Located in the first few sectors of a filesystem, this block is not used by the filesystem itself. Instead, it contains information used to boot the operating system. Only one boot block is needed per OS. While other partitions may have boot blocks, they often go unused.
- Superblock: This is a single block following the boot block that contains metadata about the filesystem, such as the size of the inode table, the size of logical blocks, and the total size of the filesystem.
- Inode table: This is the database that manages files and directories. Each file or directory has a unique entry in the inode table, which stores various attributes about it. We will cover inodes in a dedicated lesson.
- Data blocks: This is where the actual content of your files and directories is stored.
Below is an example of a disk using the MBR partition table (labeled as msdos). You can see the primary, extended, and logical partitions.
pete@icebox:~$ sudo parted -l
Model: Seagate (scsi)
Disk /dev/sda: 21.5GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 1049kB 6860MB 6859MB primary ext4 boot
2 6861MB 21.5GB 14.6GB extended
5 6861MB 7380MB 519MB logical linux-swap(v1)
6 7381MB 21.5GB 14.1GB logical xfsThis second example shows a GPT partition table, which uses unique IDs for its partitions.
Model: Thumb Drive (scsi)
Disk /dev/sdb: 4041MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 17.4kB 1000MB 1000MB first
2 1000MB 4040MB 3040MB second