---
tags:
  - Linux
  - Operating_Systems
---

# Disks
A disk is a mass storage [device](./Devices.md) which we can write to and read from.

## SCSI
* Small Computer System Interface, responsible for handling disk access on most Linux systems.
* Pronounced _scuzzy_. 
* It is a protocol that allows communicaton between printers, scanners and other peripherals in addition to harddisks. 
* The `/sda/` device that is the most common designation for the harddisk in Linux systems stands for *SCSI disk*.

## Disk schematic
The following diagram represents the basic anatomy of a disk device.

![](/img/harddisk.png)


* A disk is divided up into **partitions** which are subsections of the overall disk. The kernel presents each partition as a [block device](./Devices.md#Devices.md) 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. 
* The **filesystem** is a database of files and directories: this comprises the bulk of the partition and is what you interact with in [user space](./User_Space.md) when reading and writing data. 

 ## Partitioning disks 

### 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`.

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

```bash
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:

```bash
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](./User_Space.md#root-user-superuser). The part of the filesystem that you need sudo priveleges to access and where you manage users 
* Home dir (`/dev/nvme0n1p3`)


### Types of partition table
In general there are two types of partition table: MBR and GPT however each operating system has its own variations on these core types. The type of table used determines how the OS boots. So although partition tables also partition non-bootable sectors of a disk, they are distinguished by the boot system they enact. 

<dl>
  <dt>MBR</dt>
  <dd>Master Boot Record 
    <ul>
      <li>This is the first 512 bytes of a storage device, preceding the first partition.</li>
      <li>MBR tables allow three types of partitions: primary, extended and logical. The primary partition contains the operating system and is thus bootable. During bootstrapping, this is what is injected into memory as the kernel. The extended partition is everything else. There is only one of these however it can be broken into multiple logical partitions.  </li>
      <li>Don't understand what logical partitions are and whether they correspond to sda1, sda2 etc. 
    </ul>

  </dd>
 
  <dt>GPT</dt>

  <dd>GUID Partition Table
  
  <ul>
      <li></li>
      <li></li>
    </ul>
</dd>
</dl>

## ! To cover

What is gpt/uefi/efi
ext-4 and dos etc