Linux disk partitions

A disk is divided up into partitions which are subsections of the overall disk. The kernel presents each partition as a block device as it would with an entire disk.

The disk dedicates a small part of its contents to a partition table: this defines the different partitions that comprise the total disk space.

Viewing current partitions

Whenever you install a Linux distribution on a real or virtual machine, you must partition the drive. There are three main tools to choose from: parted, g(raphical)parted, fdisk.

For a top-level overview of your disks and their main partitions you can run lsblk (list block devices):

$ lsblk

NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda           8:0    0 465.7G  0 disk
├─sda1        8:1    0   200M  0 part
└─sda2        8:2    0 465.4G  0 part /run/media/thomas/ganesh
nvme0n1     259:0    0 476.9G  0 disk
├─nvme0n1p1 259:1    0   488M  0 part
├─nvme0n1p2 259:2    0  27.9G  0 part /
└─nvme0n1p3 259:3    0 448.5G  0 part /home

We can use parted -l to view the partition table for the current machine:

Model: SKHynix_HFS512GDE9X081N (nvme)
Disk /dev/nvme0n1: 512GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  513MB   512MB   fat32              boot, esp
 2      513MB   30.5GB  30.0GB  ext4
 3      30.5GB  512GB   482GB   ext4

We can use fdisk -l to get slightly more info:

disk /dev/nvme0n1: 476.94 GiB, 512110190592 bytes, 1000215216 sectors
Disk model: SKHynix_HFS512GDE9X081N
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 08175E77-CB9F-C34A-9032-DF29A3F8F0FE

Device            Start        End   Sectors   Size Type
/dev/nvme0n1p1     2048    1001471    999424   488M EFI System
/dev/nvme0n1p2  1001472   59594751  58593280  27.9G Linux filesystem
/dev/nvme0n1p3 59594752 1000214527 940619776 448.5G Linux filesystem

Standard Linux partitions

The two tools disclose that the main harddrive is /dev/nvme0n1 (equivalent to sda on older machines running Linux) and it has the standard three partitions:

  • Boot partition (/dev/nvme0n1p1)
  • Root dir (/dev/nvme0n1p2)
  • Home dir (/dev/nvme0n1p3)

Boot partition

  • Takes up the smallest amount of space
  • Contains the Linux kernel and the bootloader (GRUB, usually) and any other files required for booting.
  • Once the BIOS has initialized the hardware components it hands control to the bootloader stored in this partition.
  • The bootloader loads the kernel contained in its partition, into memory

Root partition (/)

  • This houses everything to do with the operation of the operating system apart from the initial booting process. Once the kernel is in memory it accesses the resources in the root partition to get the OS up and running. These resources include system libraries, utilities, system-wide configs and application software.

  • The root partition is vital for the system’s functionality. Without it, the system cannot boot or operate. It needs to be
    large enough to accommodate the operating system, installed programs, and system libraries, but it doesn’t need to store user data or
    additional applications, which can be located on other partitions.

  • This is the domain of the superuser. The part of the filesystem that you need sudo priveleges to access and where you manage users

Home partition

  • This is presented as /home/, i.e. that it is a descendent of /. But this is because the Linux file system is hierarchical. Typically /home/ is usually its own partition.
  • Separating user data and system data in this way protects user data during upgrades or reinstallation. Reinstalling or upgrading the OS can be done without affecting these files.
  • Each user has their own directory off of /home, e.g /home/thomas.
  • While the root partition (/) is essential for the operation of the OS, placing /home on a separate partition allows for better management of user data, enhancing the system’s flexibility and the user’s control over their own files, without compromising the integrity or performance of the OS. This approach exemplifies the Linux philosophy of giving the user control while maintaining a clear distinction between system
    resources and user data.

Types of partition table

In the Linux world there are two main types: MBR and GPT. The type of table used determines how the OS boots. So although partition tables are also responsible for the partitioning of non-bootable sectors of a disk, they are distinguished by the boot system they implement. If we look at the output from parted and fdisk above we see that the harddrive uses the GPT partition type.

Primary, extended and logical partitions

Most standard partition tables allow for primary, extended and logical partitions. The primary partition is the part of the harddisk that contains the operating system and is thus described as ‘bootable’ and may be called the ‘boot partition’. During the bootstrapping process this is injected into memory as the kernel.

The extended partition is basically everything other than the primary partition. This is typically subdivided into other partitions that are called logical partitions. This is because they physically reside in the same sector of the disk (the extended partition) but are treated as virtual and independent disks.

In our example above:

  • /dev/nvme0n1p1 is the primary/boot partition
  • /dev/nvme0n1p2 and /dev/nvme0n1p3 comprise the extended partition and by themselves are each logical partitions.
MBR
  • Stands for Master Boot Record
  • Uses BIOS in the boot process
  • Can only works with disks up to 2TB in size
  • Only supports 4 primary partitions. This means the number of operating systems you install is limitied to this number.
  • This is the first 512 bytes of a storage device, preceding the first partition.
GPT
  • Stands for GUID Partition Table
  • Gradually replacing MBR
  • Uses UEFI instead of BIOS
  • As name indicates, every partition on disk has its own globally-unique identifier
  • Vastly more partitions available than with MBR (dependent on operating system)
  • Offers greater recovery options and anti-corruption safeguards

Creating a Linux partition table

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