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Fdisk

lastmod 2022-06-23created 2015-07-0634 edits

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Example

fdisk -l
Show the partition setup of all disks.
fdisk /dev/sda
Enter the prompt and select the /dev/sda device.

List of commands

  • a: Set the boot partition.
  • b: Set the disklabel to BSD.
  • c: Set the DOS compatibility flag.
  • d: Delete a partition.
  • l: Show the available partition types.
  • m: Show partitioning help.
  • n: Create a new partition.
  • o: Create a new DOS partition table.
  • p: Show the current partition information.
  • q: Exit fdisk without saving the changes.
  • s: Create a SUN disklabel.
  • t: Change the partition type.
  • u: View or change the units information.
  • v: Verify the specified partition.
  • w: Save the partition information set so far and exit fdisk.
  • x: Enter partitioning expert mode.
    • In expert mode you can change the number of cylinders, the number of heads, and the number of sectors per track.

Primary, Extended, Logical partition

First, let's look at the types of partitions. Partitions can be divided into primary, extended, and logical partitions. A single hard disk can have at most 4 primary partitions. If a hard disk needs 5 or more partitions, you create an extended partition and put several logical partitions inside it. A hard disk can have exactly one extended partition. And if you create an extended partition, you can create at most 3 primary partitions. The number of logical partitions is also limited to 59. So the maximum number of partitions that can be created on one hard disk and that can store data is 62 (3 primary, 59 logical). An extended partition cannot store data. Think of the extended partition as a shell wrapping the logical partitions.

Troubleshooting

vfat signature

A message asks whether to remove the vfat signature. 

Created a new partition 1 of type 'Linux' and of size 7.3 GiB.
Partition #1 contains a vfat signature.

Do you want to remove the signature? [Y]es/[N]o:

The signature partition is basically a mark/beacon there is something there, and it is not empty. It also may identify a partition.

It is useful on the context of several utilities/OS to tell the partition has already data there.

Moving a partition size/recreating a partition is usually a non-destructive operation up to the point before formatting it.

So a signature warning is signalling you "There is already data here!...are you sure you want to go ahead?"

As for removing it or not, it depends on wether you are for instance resizing a partition or creating a partition anew. If you are creating a partition anew, obviously you may want to remove the signature, if you are resizing a partition you surely want to keep it.

The use of partition signatures is not exclusive to Linux.

From How to wipe a signature from a disk device on Linux with wipefs command 

Each disk and partition has some sort of signature and metadata/magic strings on it. The metadata used by operating system to configure disks or attach drivers and mount disks on your system

In short, it's asking "There is already data here. Do you want to remove it?"

Cannot modify the partition map

If the disk cannot be formatted on another OS, change the partition table. Use fdisk as follows. 

$ sudo fdisk /dev/sdd
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): m

Help:

  DOS (MBR)
   a   toggle a bootable flag
   b   edit nested BSD disklabel
   c   toggle the dos compatibility flag

  Generic
   d   delete a partition
   F   list free unpartitioned space
   l   list known partition types
   n   add a new partition
   p   print the partition table
   t   change a partition type
   v   verify the partition table
   i   print information about a partition

  Misc
   m   print this menu
   u   change display/entry units
   x   extra functionality (experts only)

  Script
   I   load disk layout from sfdisk script file
   O   dump disk layout to sfdisk script file

  Save & Exit
   w   write table to disk and exit
   q   quit without saving changes

  Create a new label
   g   create a new empty GPT partition table
   G   create a new empty SGI (IRIX) partition table
   o   create a new empty DOS partition table
   s   create a new empty Sun partition table

Command (m for help): o

Created a new DOS disklabel with disk identifier 0xe5a2446d.
The old iso9660 signature will be removed by a write command.

Command (m for help): p
Disk /dev/sdd: 28.7 GiB, 30752000000 bytes, 60062500 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: 0xe5a2446d

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

Partition recovery

Unlike fdisk on Windows systems, fdisk on Linux allows recovery if used incorrectly. First, write down the previous cylinder numbers before partitioning. Then, if you save a wrong partition layout, you can attach the disk to another system and match the cylinders to restore the previous one. This can also be used for security: if you write down the hard disk's cylinder numbers and partition it differently, the file system can no longer be used properly. Writing down the cylinder numbers is also a way to prepare for accidents; if you mistakenly save something wrong while running fdisk, restoring according to the previous cylinder numbers brings it back to the previous state.

See also

Favorite site

Tutorials

References

  1. Joy_and_books_-Linux_sysadmin-_fdisk.pdf ↩

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