279 lines
9.2 KiB
Markdown
279 lines
9.2 KiB
Markdown
---
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tags:
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- disks
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---
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# Disk partitions
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A disk is divided up into [partitions](Partitions.md)
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which are subsections of the overall disk. The kernel presents each partition as
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a [block device](Devices.md#Devices) as it would with an
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entire disk.
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The disk dedicates a small part of its contents to a **partition table**: this
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defines the different partitions that comprise the total disk space.
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## Viewing current partitions
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Whenever you install a Linux distribution on a real or virtual machine, you must
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partition the drive. There are three main tools to choose from: `parted`,
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`g(raphical)parted`, `fdisk`.
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For a top-level overview of your disks and their main partitions you can run
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`lsblk` (_list block devices_):
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```bash
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$ lsblk
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NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
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sda 8:0 0 465.7G 0 disk
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├─sda1 8:1 0 200M 0 part
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└─sda2 8:2 0 465.4G 0 part /run/media/thomas/ganesh
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nvme0n1 259:0 0 476.9G 0 disk
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├─nvme0n1p1 259:1 0 488M 0 part
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├─nvme0n1p2 259:2 0 27.9G 0 part /
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└─nvme0n1p3 259:3 0 448.5G 0 part /home
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```
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We can use `parted -l` to view the partition table for the current machine:
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```bash
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Model: SKHynix_HFS512GDE9X081N (nvme)
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Disk /dev/nvme0n1: 512GB
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Sector size (logical/physical): 512B/512B
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Partition Table: gpt
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Disk Flags:
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Number Start End Size File system Name Flags
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1 1049kB 513MB 512MB fat32 boot, esp
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2 513MB 30.5GB 30.0GB ext4
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3 30.5GB 512GB 482GB ext4
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```
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We can use `fdisk -l` to get slightly more info:
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```bash
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disk /dev/nvme0n1: 476.94 GiB, 512110190592 bytes, 1000215216 sectors
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Disk model: SKHynix_HFS512GDE9X081N
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Units: sectors of 1 * 512 = 512 bytes
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Sector size (logical/physical): 512 bytes / 512 bytes
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I/O size (minimum/optimal): 512 bytes / 512 bytes
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Disklabel type: gpt
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Disk identifier: 08175E77-CB9F-C34A-9032-DF29A3F8F0FE
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Device Start End Sectors Size Type
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/dev/nvme0n1p1 2048 1001471 999424 488M EFI System
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/dev/nvme0n1p2 1001472 59594751 58593280 27.9G Linux filesystem
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/dev/nvme0n1p3 59594752 1000214527 940619776 448.5G Linux filesystem
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```
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The two tools disclose that the main harddrive is `/dev/nvme0n1` (equivalent to
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`sda` on older machines running Linux) and it has the standard three partitions:
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- Boot partition (`/dev/nvme0n1p1`)
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- This takes up the smallest amount of space and exists in order to bootstrap
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the operating system: to load the kernel into memory when the machine
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starts. This is where your bootloader is stored and that will be accessed by
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the BIOS. In Linux this will be GRUB.
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- Root dir (`/dev/nvme0n1p2`)
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- This is the domain of the
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[superuser](User_Space.md#root-user-superuser). The part
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of the filesystem that you need sudo priveleges to access and where you
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manage users
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- Home dir (`/dev/nvme0n1p3`)
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- The domain of the user(s)
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## Types of partition table
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In the Linux world there are two main types: MBR and GPT. The type of table used
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determines how the OS boots. So although partition tables are also responsible
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for the partitioning of non-bootable sectors of a disk, **they are distinguished
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by the boot system they implement**. If we look at the output from `parted` and
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`fdisk` above we see that the harddrive uses the GPT partition type.
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#### Primary, extended and logical partitions
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Most standard partition tables allow for primary, extended and logical
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partitions. The primary partition is the part of the harddisk that contains the
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operating system and is thus described as 'bootable' and may be called the 'boot
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partition'. During the bootstrapping process this is injected into memory as the
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[kernel](The_Kernel.md).
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The extended partition is basically everything other than the primary partition.
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This is typically subdivided into other partitions that are called _logical_
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partitions. This is because they physically reside in the same sector of the
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disk (the extended partition) but are treated as virtual and independent disks.
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In our example above:
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- `/dev/nvme0n1p1` is the primary/boot partition
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- `/dev/nvme0n1p2` and `/dev/nvme0n1p3` comprise the extended partition and by
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themselves are each logical partitions.
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<dl>
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<dt>MBR</dt>
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<dd>
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<ul>
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<li>Stands for Master Boot Record</li>
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<li>Uses BIOS in the boot process</li>
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<li>Can only works with disks up to 2TB in size</li>
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<li>Only supports 4 primary partitions. This means the number of operating systems you install is limitied to this number.</li>
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<li>This is the first 512 bytes of a storage device, preceding the first partition.</li>
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</ul>
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</dd>
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<dt>GPT</dt>
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<dd>
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<ul>
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<li>Stands for GUID Partition Table </li>
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<li>Gradually replacing MBR</li>
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<li>Uses UEFI instead of BIOS</li>
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<li>As name indicates, every partition on disk has its own globally-unique identifier</li>
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<li>Vastly more partitions available than with MBR (dependent on operating system)</li>
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<li>Offers greater recovery options and anti-corruption safeguards</li>
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</ul>
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</dd>
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</dl>
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## Creating a partition table
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To demonstrate the process of partitioning a harddrive I am going to repartition
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an external SATA drive as if it were being primed for a fresh Linux install.
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Let's take a look at the disk in its current form:
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```
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$ fdisk -l
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Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
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Disk model: My Passport 071Aumount /dev/sda2
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Disklabel type: gpt
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Disk identifier: 9993F1BB-626C-485F-8542-3CC73BB40953
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Device Start End Sectors Size Type
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/dev/sda1 40 409639 409600 200M EFI System
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/dev/sda2 409640 976455639 976046000 465.4G Apple HFS/HFS+
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```
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(This disk was previously used as a backup disk for MacOS so in addition to the
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extended partition which has a proprietary file system type (Apple HFS) it has a
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primary partition which would load the recovery OS. In contrast to my main
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harddrive this uses the standard SCSI prototcol and thus the partitions are
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prepended with `sda`.)
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#### 1. Unmount existing partitions
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```bash
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umount /dev/sda1
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umount /dev/sda2
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```
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#### 2. Deleting the existing partitions
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```bash
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# Load the disk into fdisk
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$ sudo fdisk /dev/sda
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# Select delete and run for each partition
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Command (m for help): d
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Partition number (1,2, default 2): 1
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Partition 1 has been deleted.
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Command (m for help): d
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Selected partition 2
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Partition 2 has been deleted.
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# Verify deletion with p(rint) command
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Command (m for help): p
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Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
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Disk model: My Passport 071A
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Units: sectors of 1 * 512 = 512 bytes
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Sector size (logical/physical): 512 bytes / 512 bytes
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I/O size (minimum/optimal): 512 bytes / 512 bytes
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Disklabel type: gpt
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Disk identifier: 9993F1BB-626C-485F-8542-3CC73BB40953
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# Write the changes to disk with w(rite)
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w
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# Check execution:
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$ lsblk
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NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
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sda 8:0 0 465.7G 0 disk
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```
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So now the disk has no partitions, just the physical disk space and no
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mountpoints. We are ready to re-partition.
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#### 3. Re-partition the disk
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We are going to create a GPT partition table
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```bash
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$ sudo fdisk /dev/sda
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Command (m for help): g
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Created a new GPT disklabel (GUID: E316B1A9-6F73-FB41-8CBB-FA4E3C448F2B).
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Command (m for help): n
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Partition number (1-128, default 1):
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First sector (2048-976717790, default 2048):
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# Just press enter here
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# The last sector is where we put the actual size we want for the partition
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Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-976717790, default 976715775): +100G
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Created a new partition 1 of type 'Linux filesystem' and of size 100 GiB.
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# 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.
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Command (m for help): n
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Partition number (2-128, default 2):
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First sector (209717248-976717790, default 209717248):
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Last sector, +/-sectors or +/-size{K,M,G,T,P} (209717248-976717790, default 976715775):
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Created a new partition 2 of type 'Linux filesystem' and of size 365.7 GiB.
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# Check the partitions with p(rint)
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Command (m for help): p
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Disk /dev/sda: 465.74 GiB, 500079525888 bytes, 976717824 sectors
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Disk model: My Passport 071A
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Units: sectors of 1 * 512 = 512 bytes
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Sector size (logical/physical): 512 bytes / 512 bytes
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I/O size (minimum/optimal): 512 bytes / 512 bytes
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Disklabel type: gpt
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Disk identifier: 92739978-F7D2-5042-9758-C1429B5C8E11
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Device Start End Sectors Size Type
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/dev/sda1 2048 209717247 209715200 100G Linux filesystem
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/dev/sda2 209717248 976715775 766998528 365.7G Linux filesystem
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# Then write with w
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Command (m for help): w
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The partition table has been altered.
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Calling ioctl() to re-read partition table.
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Syncing disks.
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```
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We can now confirm out new partitions:
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```bash
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$ lsblk
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NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
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sda 8:0 0 465.7G 0 disk
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├─sda1 8:1 0 100G 0 part
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└─sda2 8:2 0 365.7G 0 part
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```
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#### Naming a partition
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By default each partition will have its GUID as its name, but you can add a
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human friendly name with:
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```bash
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sudo e2label /dev/sda1 my_human_name
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```
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> Whilst we have created our partitions we cannot yet mount them. This is
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> because we have not yet set up a filesystem on the partitions. This is the
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> next step.
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