Create partitions, file systems and swap, and mount them

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$ als gewone gebruiker# als root (of met sudo)

Examendoelen in dit hoofdstuk

Partities op MBR- en GPT-schijven tonen, maken en verwijderen
Bestandssystemen bij het opstarten koppelen via UUID of label
Nieuwe partities, logische volumes en swap toevoegen zonder dataverlies
vfat-, ext4- en xfs-bestandssystemen maken, koppelen, ontkoppelen en gebruiken

Know MBR and GPT

A partition table describes the parts of a disk. There are two kinds.

	MBR (msdos)	GPT
Max. disk size	2 TiB	8 ZiB (practically unlimited)
Partitions	4 primary (or 3 + 1 extended with logical ones)	128 by default
Backup of the table	No	Yes, at the end of the disk
Boot	BIOS	UEFI (and BIOS with a BIOS boot partition)

Onthoud

New systems and disks over 2 TiB use GPT. MBR is for old BIOS systems and small disks.

14.2

See the disks

First find the right disk. A mistake here destroys data.

$lsblk
NAME          MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
vda           252:0    0   20G  0 disk
├─vda1        252:1    0    1G  0 part /boot
└─vda2        252:2    0   19G  0 part
  ├─rhel-root 253:0    0   17G  0 lvm  /
  └─rhel-swap 253:1    0    2G  0 lvm  [SWAP]
vdb           252:16   0   10G  0 disk
$lsblk -f
#fdisk -l /dev/vdb

Device names	Disk type
`/dev/sda`, `/dev/sdb`	SATA, SAS, USB, SCSI disks
`/dev/vda`, `/dev/vdb`	KVM virtual disks (virtio)
`/dev/nvme0n1` (partitions `nvme0n1p1`)	NVMe SSDs

14.3

Understand size units

Disk sellers use powers of 1000 (GB), Linux tools often use powers of 1024 (GiB). The difference grows with the size.

Unit	Bytes
KB / KiB	1000 / 1024
MB / MiB	1000² / 1024²
GB / GiB	1000³ (1 000 000 000) / 1024³ (1 073 741 824)
TB / TiB	1000⁴ / 1024⁴

Notitie

fdisk takes +1G as 1 GiB. parted takes 1GB as 1000³ bytes and 1GiB as 1024³.

14.4

Create an MBR partition with fdisk

fdisk is a menu tool. Changes are only written when you press w.

Open the disk and create an MBR table (only on an empty disk).
```
#fdisk /dev/vdb
Command (m for help): o
```

Create a primary partition of 1 GiB.

Command (m for help): n
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-20971519, default 2048): <Enter>
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971519, default 20971519): +1G

Show the table, then write it.

Command (m for help): p
Command (m for help): w

Make sure the kernel sees the new partition.
```
#partprobe /dev/vdb
#lsblk /dev/vdb
```

fdisk key	Does
`m`	Help
`p`	Print the table
`n`	New partition
`d`	Delete a partition
`t`	Change the type (like 82 swap, 8e LVM on MBR)
`o` / `g`	New empty MBR / GPT table
`w`	Write and quit
`q`	Quit without saving

14.5

Use extended and logical partitions

MBR has only 4 slots. To get more than 4 partitions, make the 4th one extended: a container for logical partitions (numbered from 5).

Command (m for help): n
Select (default e): e
Command (m for help): n
All primary partitions are in use.
Adding logical partition 5

Let op

An extended partition holds no data itself. Use all its space for logical partitions.

14.6

Create GPT partitions with fdisk

The same fdisk, but start with g (new GPT table). There is no primary/extended question.

#fdisk /dev/vdc
Command (m for help): g
Command (m for help): n
Partition number (1-128, default 1): <Enter>
First sector (2048-20971486, default 2048): <Enter>
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971486, default 20969471): +2G
Command (m for help): t
Partition type or alias (type L to list all): swap
Command (m for help): w

GPT type aliases you can type: linux, swap, lvm, raid, uefi.

14.7

Create GPT partitions with parted

parted works in a menu or with one command line — good for scripts. It writes changes at once (no w).

#parted /dev/vdc mklabel gpt
#parted /dev/vdc mkpart data xfs 1MiB 1GiB
#parted /dev/vdc mkpart swap linux-swap 1GiB 2GiB
#parted /dev/vdc print
#parted /dev/vdc unit MiB print free

Let op

mklabel on a used disk deletes the partition table at once, without a question in command-line mode.

14.8

Create file systems

A partition needs a file system before it can store files. RHEL uses XFS by default; ext4 is the other common choice; vfat is for USB sticks and the EFI partition.

#mkfs.xfs /dev/vdb1
#mkfs.ext4 /dev/vdb2
#mkfs.vfat /dev/vdb3
#mkfs -t xfs -L data /dev/vdb1
#lsblk -f /dev/vdb

	XFS	ext4
Default on RHEL	yes	no
Grow while mounted	yes	yes
Shrink	no	yes (only unmounted)
Check/repair	`xfs_repair`	`fsck.ext4` / `e2fsck`

Let op

mkfs on the wrong device destroys its data. Check with lsblk -f first; mkfs.xfs refuses to overwrite a file system unless you add -f.

14.9

Change ext4 file system settings

tune2fs changes settings of ext4, dumpe2fs shows them all.

#tune2fs -l /dev/vdb2 | head -20
#dumpe2fs -h /dev/vdb2
#tune2fs -L backup /dev/vdb2
#tune2fs -o acl,user_xattr /dev/vdb2
#tune2fs -m 1 /dev/vdb2
#tune2fs -c 30 /dev/vdb2

Option	Changes
`-L NAME`	Label
`-o OPTS`	Default mount options
`-m N`	Reserved blocks for root (default 5 %)
`-c N` / `-i TIME`	Check after N mounts / after a time
`-U UUID`	New UUID

14.10

Change XFS file system settings

XFS has xfs_admin for label and UUID (only when unmounted) and xfs_info to show the settings.

#xfs_admin -L data /dev/vdb1
#xfs_admin -l /dev/vdb1
#xfs_info /data
#xfs_admin -U generate /dev/vdb1

14.11

Add a swap partition

Swap is disk space the kernel uses when RAM is full. Make a partition with the swap type, format it with mkswap, then switch it on.

Create the partition (type swap in fdisk, or linux-swap in parted), then format it.
```
#mkswap /dev/vdb3
```

Switch it on and check.

#swapon /dev/vdb3
#swapon --show
#free -h

Make it permanent in /etc/fstab.

/etc/fstab (line)

UUID=8f1b…   none   swap   defaults   0 0

#swapoff /dev/vdb3; swapon -a; swapon --show

14.12

Add a swap file

Without free disk space for a partition, use a file as swap.

#dd if=/dev/zero of=/swapfile bs=1M count=1024
#chmod 600 /swapfile
#mkswap /swapfile
#swapon /swapfile
#echo "/swapfile none swap defaults 0 0" >> /etc/fstab

Notitie

Use dd (not fallocate) for swap files on XFS to be safe; the file must not have holes.

14.13

Mount file systems by hand

Mounting connects a file system to a folder (the mount point). umount disconnects it.

#mkdir /data
#mount /dev/vdb1 /data
#findmnt /data
#umount /data
#mount -o ro /dev/vdb1 /data
#mount -o remount,rw /data

Let op

umount fails with "target is busy" when a process uses the folder (also your own shell). Leave the folder and check with lsof /data or fuser -vm /data.

14.14

Use UUIDs and labels

Names like /dev/vdb1 can change when you add disks. A UUID (unique ID) or a label stays the same — use them in /etc/fstab.

#blkid
/dev/vdb1: LABEL="data" UUID="4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93" TYPE="xfs" PARTUUID="…"
$lsblk -f
#mount UUID="4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93" /data
#mount LABEL=data /data

14.15

Mount at boot with /etc/fstab

Every line in /etc/fstab is a file system that is mounted at boot.

/etc/fstab

UUID=4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93  /data     xfs   defaults        0 0
LABEL=backup                               /backup   ext4  defaults,noexec 0 2
UUID=8f1b…                                 none      swap  defaults        0 0

Field	Meaning
1	What: UUID=, LABEL=, or a device
2	Where: the mount point (`none` for swap)
3	File system type
4	Mount options: `defaults`, `ro`, `noexec`, `nosuid`, `nodev`, `_netdev`, `nofail`
5	dump (backup flag): 0
6	fsck order: 0 = never; 1 = root; 2 = others (XFS: 0)

Add the line, then reload systemd and test before you reboot.

#vim /etc/fstab
#systemctl daemon-reload
#mount -a
#findmnt --verify

Examentip

A wrong fstab line can stop the boot (emergency mode). Always run mount -a and findmnt --verify after editing. On the exam, reboot at the end to prove everything mounts.

Op het werk

nofail lets the system boot even when that disk is missing. For a USB or network disk add nofail (and _netdev for network storage).

14.16

Mount with systemd mount units

systemd creates a .mount unit for every fstab line. You can also write a mount unit yourself — the file name must match the mount point.

/etc/systemd/system/data.mount

[Unit]
Description=Data disk
 
[Mount]
What=/dev/disk/by-uuid/4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93
Where=/data
Type=xfs
Options=defaults
 
[Install]
WantedBy=multi-user.target

#systemctl daemon-reload
#systemctl enable --now data.mount
$systemctl status data.mount

Onthoud

/data → data.mount; /srv/web → srv-web.mount. Check the name with systemd-escape -p --suffix=mount /srv/web.

14.17

Remember the main points

The most important facts of this chapter, in short.

Disks use MBR (old, max 4 primary partitions, 2 TiB) or GPT (up to 128 partitions, very large disks).
lsblk shows disks and partitions; fdisk and gdisk change them interactively; parted works with commands too.
Make a file system with mkfs.xfs (default on RHEL), mkfs.ext4 or mkfs.vfat.
XFS can grow but not shrink; ext4 can grow and shrink.
Swap: make it with mkswap, turn it on with swapon; a swap file must be made with dd or fallocate and get permission 600.
Mount at boot with a line in /etc/fstab: device (best a UUID from blkid), mount point, type, options, dump, fsck order.
Test fstab with mount -a (and findmnt --verify) before you reboot: a wrong line can stop the boot.
systemd can also mount with .mount units.

14.18

Learn the key terms

The words of this chapter in simple English.

Term	Meaning
Partition	A part of a disk with its own start and end.
MBR	Master Boot Record: old partition table.
GPT	GUID Partition Table: modern partition table.
Extended / logical partition	On MBR, a container partition that holds more partitions.
File system	The structure that stores files on a partition (XFS, ext4, vfat).
Mount point	The folder where a file system appears.
UUID	A unique ID of a file system, safe to use in fstab.
Label	A name you give a file system.
Swap	Disk space used as extra memory.
fstab	The file with the mounts that happen at boot.