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

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)
- This takes up the smallest amount of space and exists in order to bootstrap the operating system: to load the kernel into memory when the machine starts. This is where your bootloader is stored and that will be accessed by the BIOS. In Linux this will be GRUB.
Root dir (/dev/nvme0n1p2)
- 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 dir (/dev/nvme0n1p3)
- The domain of the user(s)

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 partition table

To demonstrate the process of partitioning a harddrive I am going to repartition an external SATA drive as if it were being primed for a fresh Linux install.

Let's take a look at the disk in its current form:

$ fdisk -l

  Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
  Disk model: My Passport 071Aumount /dev/sda2
  Disklabel type: gpt
  Disk identifier: 9993F1BB-626C-485F-8542-3CC73BB40953

  Device      Start       End   Sectors   Size Type
  /dev/sda1      40    409639    409600   200M EFI System
  /dev/sda2  409640 976455639 976046000 465.4G Apple HFS/HFS+

(This disk was previously used as a backup disk for MacOS so in addition to the extended partition which has a proprietary file system type (Apple HFS) it has a primary partition which would load the recovery OS. In contrast to my main harddrive this uses the standard SCSI prototcol and thus the partitions are prepended with sda.)

1. Unmount existing partitions

umount /dev/sda1
umount /dev/sda2

2. Deleting the existing partitions

# Load the disk into fdisk
$ sudo fdisk /dev/sda

# Select delete and run for each partition
Command (m for help): d
Partition number (1,2, default 2): 1

Partition 1 has been deleted.

Command (m for help): d
Selected partition 2
Partition 2 has been deleted.

# Verify deletion with p(rint) command
Command (m for help): p
Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
Disk model: My Passport 071A
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: 9993F1BB-626C-485F-8542-3CC73BB40953

# Write the changes to disk with w(rite)
w

# Check execution:
$ lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda           8:0    0 465.7G  0 disk

So now the disk has no partitions, just the physical disk space and no mountpoints. We are ready to re-partition.

3. Re-partition the disk

We are going to create a GPT partition table

$ sudo fdisk /dev/sda
Command (m for help): g
Created a new GPT disklabel (GUID: E316B1A9-6F73-FB41-8CBB-FA4E3C448F2B).

Command (m for help): n
Partition number (1-128, default 1): 
First sector (2048-976717790, default 2048):

# Just press enter here

# The last sector is where we put the actual size we want for the partition
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-976717790, default 976715775): +100G
Created a new partition 1 of type 'Linux filesystem' and of size 100 GiB.

# For the second and final partition, we follow the same steps but don't add a partition size, since it will default to what is left. 

Command (m for help): n
Partition number (2-128, default 2): 
First sector (209717248-976717790, default 209717248): 
Last sector, +/-sectors or +/-size{K,M,G,T,P} (209717248-976717790, default 976715775): 

Created a new partition 2 of type 'Linux filesystem' and of size 365.7 GiB.

# Check the partitions with p(rint)
Command (m for help): p
Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
Disk model: My Passport 071A
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: 92739978-F7D2-5042-9758-C1429B5C8E11

Device         Start       End   Sectors   Size Type
/dev/sda1       2048 209717247 209715200   100G Linux filesystem
/dev/sda2  209717248 976715775 766998528 365.7G Linux filesystem

# Then write with w
Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

We can now confirm out new partitions:

$ lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda           8:0    0 465.7G  0 disk 
├─sda1        8:1    0   100G  0 part 
└─sda2        8:2    0 365.7G  0 part

Whilst we have created our partitions we cannot yet mount them. This is because we have not yet set up a filesystem on the partitions. This is the next step.

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