How to Extend Logical and Extended Partitions With fdisk

1. Overview

The fdisk program is a Linux command-line utility for creating and modifying partition tables. It’s a powerful tool and can be dangerous when misused. Hence, only root or users with sudo privileges can alter the partition table using the program.

In this tutorial, we’ll look at what partitions mean. Also, we’ll learn the procedure for extending logical and extended partitions using the current version of the fdisk program.

2. Disk Partitioning

Of course, a drive requires at least one partition before we can format and store files on it. We can partition a disk into one or more logical disks, each of which operates as a separate disk and has its file system.

Also, a partition table contains the information about the partitions. The fdisk utility recognizes Sun, SGI, and BSD partition tables in addition to GPT (GUID Partition Table) and MBR (Master Boot Record). Many operating systems and disks use the GPT and MBR partition tables.

2.1. Listing Partitions

To list the partition table of a device, we use the fdisk command with the -l option and the device name. However, if we don’t specify any device, it will print partition tables of all the devices in the /proc/partitions file.

In addition, the partition consists of the device name and partition number. For example, /dev/vda1 is the first partition on the system’s primary hard drive.

Let’s take a look at the GPT partition tables:

fdisk -l

GPT (GUID Partition Table) is the modern standard partition table. It uses 64-bit logical block addresses and allows an unlimited number of partitions. However, some partitioning tools restrict the number of partitions to 128.

The GPT partitioning scheme reserves the first sector for a protective MBR in the GPT specification. Thus, it prevents MBR-only partitioning tools from misrecognizing and overwriting GPT disks.

2.2. Master Boot Record

The Master Boot Record (MBR or DOS-type) has its limitations compared to GPT. Hence, GPT is gradually replacing MBR.

The features of MBR include:

• support for only four primary partitions
• it reserves the numbers 1-4 for the primary partitions
• for more partitions, we have to make one of the primary partitions an “extended partition”
• the extended partition contains the logical partitions
• logical partitions numbering starts from 5
• it works with disks of 2 TB maximum in size

Let’s see an example of an MBR partition table:

 fdisk -l

...

Disk /dev/sda: 465.78 GiB, 500107862016 bytes, 976773168 sectors
Disk model: WDC WD5000AAKX-6
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: 0x64ec60b3

Device     Boot   Start       End   Sectors   Size Id Type
/dev/sda1  *       2048   1050623   1048576   512M  b W95 FAT32
/dev/sda2       1052670 976771071 975718402 465.3G  5 Extended
/dev/sda5       1052672   2549759   1497088   731M 83 Linux
/dev/sda6       2551808 976771071 974219264 464.6G 83 Linux

Generally, SATA device names follow the pattern /dev/sd[a-z], while NVMe device names have the pattern /dev/nvme[1-9]n[1-9]. Also, both SATA and NVMe devices support the GPT and MBR partition tables.

3. Extend Partition with fdisk

The procedure to extend an extended or logical partition with fdisk starts with deleting both partitions. Then, we recreate the partition with the desired endpoint. Using the fdisk expert menu, we can adjust the beginning of the partition to expand it, thereby adjusting the start of the partition and not the endpoint.

3.1. fdisk Command Menu

The command menu allows the use of single-letter commands to specify actions. To enter the command menu of the program, we’ll need the device name from the fdisk -l command output.

Let’s enter the command menu:

fdisk /dev/vda

Typing the letter m displays a list of the commands for use in the program.

3.2. Viewing the Partition Table

Using the p command, let’s print the current partition table to the terminal from within the command menu:

From the output, we have an extended partition and two logical partitions. Following the procedure, we’ll delete the extended and logical partitions, recreate them with our desired endpoint, and then extend the starting points.

3.3. Deleting Partitions

The d command in the command menu deletes a partition.

On entering the d command, the dialogue would require the number of the partition to delete. This is the number associated with the device partition from the p command’s output. For example, to delete the partition at /dev/vda4, we’ll key in 4.

However, deleting the extended partition would remove all the logical partitions under it. Again, if we don’t specify any partition number in the dialogue, the program defaults to the last logical partition.

Let’s delete the extended partition:

The command dialogue 2 deletes both the extended and logical partition. After deleting the partition, we can print and confirm our changes by viewing the current partition table:

Next, we’ll recreate the extended and logical partition.

3.4. Creating a Partition

The n command creates a new primary or extended partition. Again, the program will default to a primary partition if no option is selected.

Also, fdisk will number the new partition as /dev/vda4 for this example:

Next, we’ll specify the starting sector of the new partition on the disk. However, pressing Enter accepts the program’s default sector – the first free sector on the disk.

Lastly, we’ll set the last sector of the partition on the disk. Again, we’ll hit Enter to use up all available space after the initial sector.

Also, we can specify a size for the partition. For instance, +10G would result in a ten-gigabyte partition or +800M for an 800-megabyte partition. However, if we don’t specify a unit after the + sign, fdisk uses sectors as the unit. For example, specifying +10000 in the dialogue would result in a partition with 10000 sectors from start to end.

Let’s set the first and last sectors of the partition:

Subsequently, we’ll create a new logical partition. It will begin at the end of /dev/vda1 and take the remaining space of the extended partition. Also, creating a logical partition requires that an extended partition is present.

To illustrate, let’s create a logical partition. The procedure is the same as that of the extended partition. In contrast, the command for creating the logical partition type is l.

Since the logical partition will take up the entire extended partition, we’ll choose the default first and last sectors the program specifies:

To confirm our change, let’s run the p command:

Next, let’s extend the logical partition. We’ll adjust its starting sector in the fdisk expert. The procedure is the same for extending an extended partition.

4. Activating New Partition

Having created the partitions, the next step is to format and mount them to the system directory tree. However, we don’t need to mount the extended partition directly. We’ll only mount the logical partition since the logical partition is inside the extended partition.

4.1. Formatting Partitions

At this point, we are ready to format the partitions for use. We’ll use ext4 for the partition in this illustration,  although one can still use any partition format.

Let’s format both partitions to ext4:

mkfs.ext4 -F /dev/vda4; mkfs.ext4 -F /dev/vda5

The flag -F would force mkfs to create a filesystem. The option is handy if the specified device is not a partition on a device or if other parameters don’t make sense.

Additionally, to forcefully create a filesystem where the filesystem appears to be in use or is mounted, we can specify the -F option twice, but it’s risky.

4.2. Mounting Partitions

To mount a partition, we’ll first create the mount point with mkdir. Then we’ll mount the partition using the new directory as the mount point.

Now, let’s create the mount point in the /mnt directory:

mkdir -p /mnt/greencloud

The -p switch ensures the creation of the parent directory if it’s not available.

Next, let’s mount the new partition to the new directory:

mount /dev/vda4 /mnt/greencloud

Lastly, partitions will stay mounted until we unmount them or shutdown the machine. We can define the mount in the /etc/fstab file to automatically mount a partition when the Linux system boots up.

5. Conclusion

In this article, we’ve learned how to delete, create, and extend partitions in a Linux system from the command line. Also, we’ve seen how to format and mount the partitions after creating a new partition.

However, editing partitions in use can lead to harmful damage. Hence, to edit system partitions, it’s better to back up its content and boot from a Live CD or bootable USB stick. Also, we have to carry out this task carefully when we do it on a public server and note changes to system files for future admins to reference.