Disks and partitions

Everything you store lives on a disk somewhere, but a disk by itself is just an empty row of numbered blocks. Before it can hold files you divide it into partitions, put a file system on each, and mount them into the directory tree. This chapter walks through that sequence, then through the daily problems of storage: making it permanent, running out of space, and checking for damage. The first step is understanding what the system calls a disk.

Explain block devices, disks and partitions, and recognise device names like sda, vda and nvme0n1p1.
Compare GPT and MBR, and list the disks and partitions with lsblk, blkid and parted.
Create a GPT partition table and partitions with parted -s.

Block devices

A block device is something the kernel reads and writes in fixed-size blocks: disks, partitions, USB sticks, virtual disks. They appear as files in /dev:

Name	What it is
`/dev/sda`, `/dev/sdb`, …	SCSI/SATA/USB disks and many virtual disks, in the order found
`/dev/vda`, …	virtio virtual disks
`/dev/nvme0n1`	The first NVMe disk (its partitions are `nvme0n1p1`, `nvme0n1p2`, …)
`/dev/sda1`, `/dev/sda2`	Partitions 1 and 2 of `sda`

Plain letters name disks, numbers name partitions. The order of letters can change when disks are added, which is why file systems are identified by UUID when it matters.

What is on this machine

[root@servera ~]# lsblk
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sda      8:0    0   20G  0 disk
├─sda1   8:1    0  100M  0 part /boot/efi
└─sda2   8:2    0 19.9G  0 part /
sdb      8:16   0    5G  0 disk
[root@servera ~]# lsblk -f
NAME   FSTYPE FSVER LABEL  UUID                                 FSAVAIL FSUSE% MOUNTPOINTS
sda
├─sda1 vfat   FAT32 UEFI   76E8-0192                              90.8M     8% /boot/efi
└─sda2 ext4   1.0   rootfs 4bbdcf8d-863d-4050-957c-c815ca3838b8   17.5G     9% /
sdb

sda is the system disk, with a small EFI boot partition and a root partition. sdb is a spare 5 GiB disk with nothing on it: it is the one you will practise on in the exercises of this chapter. Anything you do to /dev/sdb destroys its content, so always check the device name before you run a command. lsblk -f adds the file system type, label, UUID and mount point.

Partition tables

A partition table at the start of the disk lists where each partition begins and ends. Two formats exist:

	GPT	MBR (DOS)
Disk size	Larger than 2 TiB is fine	Up to 2 TiB
Partitions	Up to 128, all equal	4 primary (or 3 + an extended one)
Used by	Modern systems, UEFI	Old BIOS systems
Resilience	A copy at the end of the disk	One copy

Use GPT for new disks. Tools: parted (can be scripted), fdisk and gdisk (interactive, with a text menu: n new, p print, d delete, w write). parted -s takes commands directly, which is convenient and good for exercises.

One disk, three partitions: Select a part of /dev/sdb.

Creating partitions with parted

[root@servera ~]# parted -s /dev/sdb print
Error: /dev/sdb: unrecognised disk label
...
Partition Table: unknown
[root@servera ~]# parted -s /dev/sdb mklabel gpt
[root@servera ~]# parted -s /dev/sdb mkpart data xfs 1MiB 1025MiB
[root@servera ~]# parted -s /dev/sdb mkpart logs ext4 1025MiB 2049MiB
[root@servera ~]# parted -s /dev/sdb mkpart swap linux-swap 2049MiB 2561MiB
[root@servera ~]# parted -s /dev/sdb print
Number  Start   End     Size    File system  Name  Flags
 1      1049kB  1075MB  1074MB               data
 2      1075MB  2149MB  1074MB               logs
 3      2149MB  2685MB  537MB                swap  swap
[root@servera ~]# udevadm settle; lsblk /dev/sdb
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sdb      8:16   0    5G  0 disk
├─sdb1   8:17   0    1G  0 part
├─sdb2   8:18   0    1G  0 part
└─sdb3   8:19   0  512M  0 part

mklabel gpt writes the table; each mkpart NAME FSTYPE START END creates a partition. The FSTYPE word is only a hint stored in the table: it does not create a file system. Starting at 1MiB keeps the partition aligned to the disk's blocks. udevadm settle waits until the device files /dev/sdb1 and so on have appeared.

If lsblk does not show new partitions on a disk that is in use, partprobe /dev/sdb asks the kernel to re-read the table.

The whole picture

A new disk, step by step: Every disk follows the same four steps.

You have done step 1. The next lessons do steps 2 to 4.

Try it

Check your understanding

What is /dev/sdb2?
Which partition table type supports disks larger than 2 TiB?
After creating a partition, what is still missing before you can store files?
Which command shows disks, partitions and their file systems and mount points as a tree?
Why start the first partition at 1MiB?
Which command writes a partition table without prompts, for scripts?