102.1 Design hard disk layout

Weight: 2

Description: Candidates should be able to design a disk partitioning scheme for a Linux system.

Key Knowledge Areas

• Allocate filesystems and swap space to separate partitions or disks
• Tailor the design to the intended use of the system
• Ensure the /boot partition conforms to the hardware architecture requirements for booting
• Knowledge of basic features of LVM

The following is a partial list of the used files, terms, and utilities:

• / (root) filesystem
• /var filesystem
• /home filesystem
• /boot filesystem
• EFI System Partition (ESP)
• Swap space
• Mount points
• Partitions

Basics

Like any contemporary OS, Linux uses files and directories to operate. But unlike Windows, it does not use A:, C:, D:, etc. In Linux, everything is in *one big tree, starting with / (called root). Any partition, disk, CD, USB, network drive, ... will be placed somewhere in this huge tree.

Note: Most of external devices (USB, CD, ...) are mounted at /media/ or /mnt/ .

Unix directories

This might be your enlightening moment in your Linux journey. Understanding Filesystem Hierarchy Standard (FHS) can help you find your programs, configs, logs and ... without having prior knowledge about them. This is standard and the latest revision is for 2015.

Partitions

In the Linux world, devices are defined at /dev/. First SATA or SCSI disks you will have /dev/sda, For newer NVME drives you can see /dev/nvme0 and partitions are available as /dev/nvme0n1, and for the 3rd PATA (super old) disk you will see /dev/hdc, also for SD/eMMC/bare NAND/NOR devices you will have /dev/mmcblk0 and partitions are seen as /dev/mmcblk0p0.

You have to PARTITION the disks, that is create smaller parts on a big disk. These are self-contained sections on the main drive. OS sees these as standalone disks. We call them /dev/sda1 (first partition of the first SCSI disk) or /dev/hdb3 (3rd partition on the second disk).

BIOS systems were using MBR and could have up to 4 partitions on each disk, although instead of creating 4 Primary partitions, you could create an Extended partition and define more Logical partitions inside it.

Note: an Extended partition is just an empty box for creating Logical partitions inside it.

So:

• /dev/sda3 is the 3rd primary partition on the first disk
• /dev/sdb5 is the first logical partition on the second disk
• /dev/sda7 is the 3rd logical partition of the first physical disk

UEFI systems use GUID Partition Table (GPT) which supports 128 partitions on each device.

If you define an extended partition on a BIOS system, that will be /dev/sdx5 (1-4 for primary, and the first extended will be 5).

Linux systems can mount these partitions on different paths. Say you can have a separated disk with one huge partition for your /home and another one for your /var/logs/.

# fdisk /dev/sda
Welcome to fdisk (util-linux 2.25.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): p
Disk /dev/sda: 298.1 GiB, 320072933376 bytes, 625142448 sectors
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: dos
Disk identifier: 0x000beca1

Device     Boot     Start       End   Sectors   Size Id Type
/dev/sda1  *         2048  43094015  43091968  20.6G 83 Linux
/dev/sda2        43094016  92078390  48984375  23.4G 83 Linux
/dev/sda3        92080126 625141759 533061634 254.2G  5 Extended
/dev/sda5        92080128 107702271  15622144   7.5G 82 Linux swap / Solaris
/dev/sda6       107704320 625141759 517437440 246.8G 83 Linux

The newer GUID Partition Table (or GPT) solves these problems. If you format your disk with GPT you can have 128 primary partitions (no need for extended and logical).

Commands

parted

jadi@funlife:~$ sudo parted /dev/sda p
Model: ATA ST320LT000-9VL14 (scsi)
Disk /dev/sda: 320GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type      File system     Flags
 1      1049kB  22.1GB  22.1GB  primary   ext4            boot
 2      22.1GB  47.1GB  25.1GB  primary   ext4
 3      47.1GB  320GB   273GB   extended
 5      47.1GB  55.1GB  7999MB  logical   linux-swap(v1)
 6      55.1GB  320GB   265GB   logical

fdisk

# sudo fdisk /dev/sda
[sudo] password for jadi:

Welcome to fdisk (util-linux 2.25.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): p
Disk /dev/sda: 298.1 GiB, 320072933376 bytes, 625142448 sectors
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: dos
Disk identifier: 0x000beca1

Device     Boot     Start       End   Sectors   Size Id Type
/dev/sda1  *         2048  43094015  43091968  20.6G 83 Linux
/dev/sda2        43094016  92078390  48984375  23.4G 83 Linux
/dev/sda3        92080126 625141759 533061634 254.2G  5 Extended
/dev/sda5        92080128 107702271  15622144   7.5G 82 Linux swap / Solaris
/dev/sda6       107704320 625141759 517437440 246.8G 83 Linux

Note: parted does not understands GPT

gparted

A graphical tool for managing disks and partitions.

LVM

In many cases, you need to resize your partitions or even install new disks and add them to your current mount points; Increasing the total size. LVM is designed for this.

LVM helps you create one partition from different disks and add or remove space to them. The main concepts are:

• Physical Volume (PV): A whole drive or a partition. It is better to define partitions and not use whole disks - unpartitioned.
• Volume Groups (VG): This is the collection of one or more PVs. OS will see the vg as one big disk. PVs in one VG, can have different sizes or even be on different physical disks.
• Logical Volumes (LV): OS will see lvs as partitions. You can format an LV with your OS and use it.

Design Hard disk layout

Disk layout and allocation partitions to directories depend on your usage. First, we will discuss swap and boot and then will see three different cases.

swap

Swap in Linux works like an extended memory. The Kernel will page memory to this partition/file. It is enough to format one partition with swap file system and define it in /etc/fstab (you will see this later in 104 modules).

Note: There is no strict formula for swap size. People used to say "double the ram but not more than 8GB". On recent machines with SSDs, some say "RAM + 2" (Hibernation + some extra ) or "RAM * 2" depending on your usage.

/boot

Older Linux systems were not able to handle HUGE disks during the boot (say Terabytes) so there was a separated /boot. It is also useful to recover broken systems or even you can make /boot read-only. Most of the time, having 100MB for /boot is enough. This can be a different disk or a separated partition.

This partition should be accessible by BIOS/UEFI during the boot (No network drive).

On UEFI systems, there is a /boot/efi mount point called EFI (Extensible Firmware Interface) system partition or ESP. This contains the bootloader and kernel and should be accessible by the UEFI firmware during the boot.

Case one: Desktop computer

On a desktop computer, it is good to have one swap, one /boot, and allocate all other space to / (root).

Case two: Network workstation

As any other system /boot should be local (a physical disk is connected to the machine) and most of the time, the / (root file system) is also local. But in a network station, /home can be mounted from a network drive (NFS, SMB, SSH, ..). This lets users sit at any station, login, and have their own home mounted from a network drive. Swap can be mounted from network or local.

Case three: Server

On servers /boot is still local and based on usage, /home can be local or network. In many cases, we separate the /var because logs and many other files are there and being updated so it is good to separate it or even put it on more advanced storage (like RAID disks to prevent data loss). Some people also separate the /usr and write-protect it (read-only file systems) or even mount the /usr from the network so they can change/update one file on the network storage and all the servers will use the new file (You remember? /usr contains important executables like Apache webserver).

Bonus! know about zram

Here I'll review the 3 different methods to add a swap to your OS. We will have a look at 3 different distros:

• Debian 11: Uses a swap partition
• Ubuntu 22.04: Uses a swap file
• Fedora 36: uses zram In short, zram is a virtual disk on your RAM which can be used as swap space or be mounted anywhere you like; A common example is \tmp. Let's see.