Disks, Partitions, and Filesystems

One tree, no drive letters

Windows gives each drive its own letter and its own tree: C:\, D:\. Linux has one directory tree, starting at /, and every disk’s file system is attached at some folder in it, a mount point ¹. A USB drive doesn’t become E:; it appears as a folder like /media/me/backup. Attaching a file system to the tree is mounting it ¹.

Devices and partitions

Each disk is a device file under /dev. The kernel treats SATA disks, USB drives, and similar devices alike, naming them sda, sdb, and so on, and their partitions sda1, sda2 ¹. NVMe SSDs, common in laptops, are nvme0n1, with partitions nvme0n1p1, nvme0n1p2; SD cards are mmcblk0 with mmcblk0p1 ².

A disk is divided into partitions by a partition table at its start. Current computers use GPT; older ones and many USB sticks use the older MBR (also called msdos) ². Each partition holds one file system, the structure that turns raw blocks into files and folders.

lsblk lists them all, mounted or not ^{1 3}. From TLCL, on a desktop with two disks and a DVD drive:

[me@linuxbox ~]$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda      8:0    0 111.8G  0 disk
├─sda1   8:1    0   976M  0 part /boot/efi
│                                /
└─sda3   8:3    0  14.9G  0 part [SWAP]
sdb      8:16   0 931.5G  0 disk
├─sdb1   8:17   0 922.2G  0 part /home
└─sdb2   8:18   0   9.3G  0 part
sr0     11:0    1  1024M  0 rom

Read it as a tree: disk sda holds partitions mounted at /boot/efi and /, plus swap; disk sdb holds /home. RM is 1 for removable devices ¹. lsblk -f shows file system type, label, UUID, and free space instead ³. On Ubuntu you’ll also see many loop devices; those are snaps, each mounted as a small read-only file system.

A typical single-disk Ubuntu install has a small EFI system partition formatted vfat, mounted at /boot/efi, which holds the boot loader, and a large ext4 partition mounted at /, holding everything else ¹.

Which file system

• ext4: the standard Linux file system and Ubuntu’s default. It keeps Linux owners and permissions, which matters for the system and for backups ¹.
• exFAT and FAT32 (Linux calls FAT vfat): for USB sticks and SD cards shared with Windows, Macs, and cameras ⁴. They don’t store Linux permissions, and the ArchWiki notes exFAT has no journal, so it’s for carrying files, not for keeping them safe ⁴.
• NTFS: Windows’ file system. Linux reads and writes it, useful for a Windows partition or drive ¹.

Labels and UUIDs

Device names come from the order the kernel detects hardware, so they can change: plug in another drive and yesterday’s sdb is today’s sdc ⁵. A file system carries two names of its own that don’t change:

• a UUID, a long unique ID made when it was formatted;
• a label, a short name you choose, like backup.

me@garden-laptop:~$ lsblk -f
me@garden-laptop:~$ sudo blkid /dev/sda1

blkid prints a device’s type, label, and UUID ⁶. Anywhere you configure a drive, such as /etc/fstab in the next lesson, use LABEL= or UUID= rather than a /dev name ⁵.

Preparing a backup drive

The Shell course’s backup needs an ext4 drive: rsync --link-dest relies on Linux ownership and permissions, which Windows file systems don’t keep. Here’s how to turn an external drive into one. This erases the drive. TLCL’s warning applies to every step: use the device name your system shows, never one from an example, or you could format the wrong disk ¹.

1. Find it, carefully. Run lsblk with the drive unplugged, plug it in, and run it again; the new entry is your drive. Check its size matches ¹. In what follows it’s sdX; substitute the real name.

2. Unmount it, since the desktop has probably mounted it: sudo umount /dev/sdX1 (one per mounted partition).

3. Make a fresh partition table and one partition, with parted ^{1 7}:

me@garden-laptop:~$ sudo parted /dev/sdX
(parted) mklabel gpt
(parted) mkpart backup ext4 0% 100%
(parted) print
(parted) quit

mklabel gpt writes a new, empty GPT partition table; mkpart makes one partition, here named backup, from 0% to 100% of the drive; print shows the result before you leave ⁷. Changes happen as you type each command, and parted takes the whole disk, /dev/sdX, not a partition ^{1 7}.

4. Make the file system, with the label backup ^{1 8}:

me@garden-laptop:~$ sudo mkfs -t ext4 -L backup /dev/sdX1

TLCL’s run of the same command on a 120 GB partition ¹:

mke2fs 1.46.5 (30-Dec-2021)
Creating filesystem with 29296640 4k blocks and 7331840 inodes
Filesystem UUID: 3365d135-d8d9-4b93-a57a-ec3567c8548a
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632,
    2654208, 4096000, 7962624, 11239424, 20480000, 23887872
Allocating group tables: done
Writing inode tables: done
Creating journal (131072 blocks): done
Writing superblocks and filesystem accounting information: done

By default ext4 sets aside 5% of the space for root, to keep the system working when a disk fills up ⁸. On a drive used only for backups, -m 1 keeps that to 1%.

The Disks app on the Ubuntu desktop does all of this with buttons, under the menu for the drive and then the gear for the partition. It’s the same steps, and the same need to pick the right drive.

5. Unplug and replug. The desktop mounts it at /media/<you>/backup, named after the label ¹. The new file system’s top folder belongs to root, because mkfs ran as root ⁸; make it yours once with sudo chown $USER: /media/$USER/backup.

Unmounting, and fsck

Linux holds data waiting to be written in memory, to keep things fast. Unmounting writes it all out; pulling a drive without unmounting can lose data or damage the file system ¹. Use Eject in the file manager, or:

me@garden-laptop:~$ sudo umount /media/me/backup

If it refuses because the drive is busy, something is still using it: open files, or a terminal whose current directory is on the drive. cd elsewhere and try again ^{1 9}.

If a file system was damaged, say by a power cut, fsck checks and repairs it, run on an unmounted file system ¹:

me@garden-laptop:~$ sudo fsck /dev/sdX1

A healthy one reports clean ¹. You’ll rarely need it by hand: the system checks file systems at boot, and corruption is rare without failing hardware ¹.

Your turn

So

Linux has one directory tree; every file system is mounted at a folder in it. Disks appear as sda, nvme0n1, and so on, partitions as sda1 or nvme0n1p1, and lsblk (or lsblk -f) lists them all. Use ext4 for Linux and backups, exFAT or FAT for sticks shared with other devices, NTFS for Windows drives. Device names can change, so identify file systems by label or UUID. To prepare a backup drive, find it with lsblk before and after plugging it in, parted a GPT table and one partition, mkfs -t ext4 -L backup, and chown its top folder. Unmount before unplugging; fsck repairs unmounted file systems.