disk partitions

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

This chapter continues on the hard disk devices from the previous one. Here we will put partitions on those devices.

This chapter prepares you for the next chapter, where we put file systems on our partitions.

about partitions

primary, extended and logical

Linux requires you to create one or more partitions. The next paragraphs will explain how to create and use partitions.

A partition's geometry and size is usually defined by a starting and ending cylinder (sometimes by sector). Partitions can be of type primary (maximum four), extended (maximum one) or logical (contained within the extended partition). Each partition has a type field that contains a code. This determines the computer's operating system or the partitions file system.

Partition Type	naming
Primary (max 4)	1-4
Extended (max 1)	1-4
Logical	5-

partition naming

We saw before that hard disk devices are named /dev/sdx with x depending on the hardware configuration. Next is the partition number, starting the count at 1. Hence the four (possible) primary partitions are numbered 1 to 4. Logical partition counting always starts at 5. Thus /dev/sda2 is the second partition on the first SATA hard disk device, and /dev/sdb5 is the first logical partition on the second SATA hard disk device.

partition	device
`/dev/sda1`	first primary partition on `/dev/sda`
`/dev/sda2`	second primary or extended partition on `/dev/sda`
`/dev/sda5`	first logical drive on `/dev/sda`
`/dev/sdb6`	second logical on `/dev/sdb`

The picture below shows two (spindle) disks with partitions. Note that an extended partition is a container holding logical drives.

discovering partitions

fdisk -l

In the fdisk -l example below you can see that two partitions exist on /dev/sdb. The first partition spans 31 cylinders and contains a Linux swap partition. The second partition is much bigger.

student@linux:~$ sudo fdisk -l /dev/sdb

Disk /dev/sdb: 100.0 GB, 100030242816 bytes
255 heads, 63 sectors/track, 12161 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot    Start      End     Blocks   Id  System
/dev/sdb1             1       31     248976   82  Linux swap / Solaris
/dev/sdb2            32    12161   97434225   83  Linux

/proc/partitions

The /proc/partitions file contains a table with major and minor number of partitioned devices, their number of blocks and the device name in /dev. Verify with /proc/devices to link the major number to the proper device.

student@linux:~$ cat /proc/partitions 
major minor  #blocks  name

3     0     524288 hda
3    64     734003 hdb
8     0    8388608 sda
8     1     104391 sda1
8     2    8281507 sda2
8    16    1048576 sdb
8    32    1048576 sdc
8    48    1048576 sdd
253     0    7176192 dm-0
253     1    1048576 dm-1

The major number corresponds to the device type (or driver) and can be found in /proc/devices. In this case 3 corresponds to ide and 8 to sd. The major number determines the device driver to be used with this device.

The minor number is a unique identification of an instance of this device type. The devices.txt¹ file in the Linux kernel source tree contains a full list of major and minor numbers.

parted and others

You may be interested in alternatives to fdisk like parted, cfdisk, sfdisk and gparted. This course mainly uses fdisk to partition hard disks.

parted is recommended by some Linux distributions for handling storage with gpt instead of mbr.

Below a screenshot of parted on Enterprise Linux.

[vagrant@el ~]$ rpm -q parted
parted-3.5-2.el9.x86_64
[vagrant@el ~]$ sudo parted /dev/sdb
GNU Parted 3.5
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 21.5GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  5370MB  5369MB  primary  ext4
 2      5370MB  10.7GB  5369MB  primary  xfs
 3      10.7GB  16.1GB  5369MB  primary  ext3
 4      16.1GB  21.5GB  5368MB  primary  ntfs

(parted) quit

partitioning new disks

In the example below, we bought a new disk for our system. After the new hardware is properly attached, you can use fdisk or parted to create the necessary partition(s). This example uses fdisk, but there is nothing wrong with using parted.

recognising the disk

First, we check with fdisk -l whether Linux can see the new disk. Yes it does, the new disk is seen as /dev/sdb, but it does not have any partitions yet.

student@linux:~$ sudo fdisk -l

Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start      End      Blocks   Id  System
/dev/sda1   *       1        13      104391   83  Linux
/dev/sda2          14      1566    12474472+  8e  Linux LVM

Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/sdb doesn't contain a valid partition table

opening the disk with fdisk

Then we create a partition with fdisk on /dev/sdb. First we start the fdisk tool with /dev/sdb as argument. Be very very careful not to partition the wrong disk!!

student@linux:~$ fdisk /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI...
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected...

empty partition table

Inside the fdisk tool, we can issue the p command to see the current disks partition table.

Command (m for help): p

Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start         End      Blocks   Id  System

create a new partition

No partitions exist yet, so we issue n to create a new partition. We choose p for primary, 1 for the partition number, 1 for the start cylinder and 14 for the end cylinder.

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-130, default 1): 
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-130, default 130): 14

We can now issue p again to verify our changes, but they are not yet written to disk. This means we can still cancel this operation! But it looks good, so we use w to write the changes to disk, and then quit the fdisk tool.

Command (m for help): p

Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1          14      112423+  83  Linux

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

display the new partition

Let's verify again with fdisk -l to make sure reality fits our dreams. Indeed, the screenshot below now shows a partition on /dev/sdb.

student@linux:~$ sudo fdisk -l

Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start        End      Blocks   Id  System
/dev/sda1   *        1         13      104391   83  Linux
/dev/sda2           14       1566    12474472+  8e  Linux LVM

Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1          14      112423+  83  Linux

about the partition table

master boot record

The partition table information (primary and extended partitions) is written in the master boot record or mbr. You can use dd to copy the mbr to a file.

This example copies the master boot record from the first SATA hard disk.

dd if=/dev/sda of=/SCSIdisk.mbr bs=512 count=1

The same tool can also be used to wipe out all information about partitions on a disk. This example writes zeroes over the master boot record.

dd if=/dev/zero of=/dev/sda bs=512 count=1

Or to wipe out the whole partition or disk.

dd if=/dev/zero of=/dev/sda

partprobe

Don't forget that after restoring a master boot record with dd, that you need to force the kernel to reread the partition table with partprobe. After running partprobe, the partitions can be used again.

[student@linux ~]$ sudo partprobe

logical drives

The partition table does not contain information about logical drives. So the dd backup of the mbr only works for primary and extended partitions. To backup the partition table including the logical drives, you can use sfdisk.

This example shows how to backup all partition and logical drive information to a file.

sfdisk -d /dev/sda > parttable.sda.sfdisk

The following example copies the mbr and all logical drives info from /dev/sda to /dev/sdb.

sfdisk -d /dev/sda | sfdisk /dev/sdb

GUID partition table

gpt was developed because of the limitations of the 1980s mbr partitioning scheme (for example only four partitions can be defined, and they have a maximum size two terabytes).

Since 2010 gpt is a part of the uefi specification, but it is also used on bios systems.

Newer versions of fdisk work fine with gpt, but most production servers today (mid 2015) still have an older fdisk.. You can use parted instead.

labeling with parted

parted is an interactive tool, just like fdisk. Type help in parted for a list of commands and options.

This screenshot shows how to start parted to manage partitions on /dev/sdb.

[student@linux ~]$ sudo parted /dev/sdb
GNU Parted 3.1
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted)

Each command also has built-in help. For example help mklabel will list all supported labels. Note that we only discussed mbr (msdos) and gpt in this book.

(parted) help mklabel
    mklabel,mktable LABEL-TYPE             create a new disklabel (partition table)

    LABEL-TYPE is one of: aix, amiga, bsd, dvh, gpt, mac, msdos, pc98, sun, loop

We create an mbr label.

(parted) mklabel msdos>
Warning: The existing disk label on /dev/sdb will be destroyed and all data on
this disk will be lost. Do you want to continue?
Yes/No? yes
(parted) mklabel gpt
Warning: The existing disk label on /dev/sdb will be destroyed and all data on
this disk will be lost. Do you want to continue?
Yes/No? Y

partitioning with parted

Once labeled it is easy to create partitions with parted. This screenshot starts with an unpartitioned (but gpt labeled) disk.

(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 8590MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start  End  Size  File system  Name  Flags

(parted)

This example shows how to create two primary partitions of equal size.

(parted) mkpart primary 0 50%
Warning: The resulting partition is not properly aligned for best performance.
Ignore/Cancel? I
(parted) mkpart primary 50% 100%
(parted)

Verify with print and exit with quit. Since parted works directly on the disk, there is no need to w(rite) like in fdisk.

(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 8590MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name     Flags
    1      17.4kB  4295MB  4295MB               primary
    2      4295MB  8589MB  4294MB               primary

(parted) quit
Information: You may need to update /etc/fstab.

practice: partitions

Use fdisk -l to display existing partitions and sizes.
Use df -h to display existing partitions and sizes.
Compare the output of fdisk and df.
Create a 200MB primary partition on a small disk.
Create a 400MB primary partition and two 300MB logical drives on a big disk.
Use df -h and fdisk -l to verify your work.
Compare the output again of fdisk and df. Do both commands display the new partitions ?
Create a backup with dd of the mbr that contains your 200MB primary partition.
Take a backup of the partition table containing your 400MB primary and 300MB logical drives. Make sure the logical drives are in the backup.
(optional) Remove all your partitions with fdisk. Then restore your backups.

solution: partitions

Use fdisk -l to display existing partitions and sizes.

sudo fdisk -l
Use df -h to display existing partitions and sizes.

df -h
Compare the output of fdisk and df.

Some partitions will be listed in both outputs (maybe /dev/sda1 or /dev/hda1).

Create a 200MB primary partition on a small disk.

Choose one of the disks you added (this example uses /dev/sdc).

student@linux ~$ sudo fdisk /dev/sdc
...
Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-261, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (1-261, default 261): +200m
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.

Create a 400MB primary partition and two 300MB logical drives on a big disk.

Choose one of the disks you added (this example uses /dev/sdb)
```
student@linux ~$ sudo fdisk /dev/sdb
```
inside fdisk:
- primary partition: n p 1 +400m enter
- extended partition: n e 2 enter enter
- logical partitions (twice): n l +300m enter
Use df -h and fdisk -l to verify your work.

fdisk -l ; df -h
Compare the output again of fdisk and df. Do both commands display the new partitions ?

The newly created partitions are visible with fdisk.

But they are not displayed by df.
Create a backup with dd of the mbr that contains your 200MB primary partition.
```
dd if=/dev/sdc of=bootsector.sdc.dd count=1 bs=512
```
Take a backup of the partition table containing your 400MB primary and 300MB logical drives. Make sure the logical drives are in the backup.
```
sfdisk -d /dev/sdb > parttable.sdb.sfdisk
```

See e.g. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/devices.txt?h=v6.12 ↩