Disks and Partitioning

Disk and partition management is a core skill for Linux system administrators. This article starts with viewing disk information and covers MBR/GPT partition creation, filesystem formatting, mount operations, and automatic mounting at boot via /etc/fstab.

Viewing Disk and Block Device Information

Viewing Block Devices with lsblk

lsblk lists all block devices and their mount points in a tree structure:

lsblk

Example output:

NAME          MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sda             8:0    0   50G  0 disk
├─sda1          8:1    0    1G  0 part /boot
└─sda2          8:2    0   49G  0 part
  ├─rl-root   253:0    0   45G  0 lvm  /
  └─rl-swap   253:1    0    4G  0 lvm  [SWAP]
sdb             8:16   0  100G  0 disk

For more detailed information including filesystem type and UUID:

lsblk -f

Listing the Partition Table with fdisk

sudo fdisk -l /dev/sdb

Viewing UUIDs with blkid

sudo blkid

Tip

UUIDs (Universally Unique Identifiers) do not change when disk order changes. It is recommended to use UUIDs instead of device names in /etc/fstab.

MBR Partitioning: Using fdisk

fdisk is used to manage MBR (Master Boot Record) partition tables, supporting a maximum of 4 primary partitions or 3 primary partitions plus 1 extended partition. MBR supports disks up to 2TB.

1. Start fdisk in interactive mode:

   sudo fdisk /dev/sdb

2. Create a new partition in the interactive interface:

   Command (m for help): n
   Partition type
      p   primary (0 primary, 0 extended, 4 free)
      e   extended (container for logical partitions)
   Select (default p): p
   Partition number (1-4, default 1): 1
   First sector (2048-209715199, default 2048): Press Enter
   Last sector ... (default 209715199): +20G

3. View the created partitions:

   Command (m for help): p

4. Write the partition table and exit:

   Command (m for help): w

Common fdisk Interactive Commands

Command	Description
n	Create a new partition
d	Delete a partition
p	Print the partition table
t	Change the partition type
l	List known partition types
w	Write changes and exit
q	Quit without saving

GPT Partitioning: Using gdisk

GPT (GUID Partition Table) overcomes the limitations of MBR, supporting disks larger than 2TB and up to 128 partitions. GPT is strongly recommended for modern servers.

fdisk (util-linux 2.23+, shipped by EL8/9/10) has supported GPT since 2013 and new disks default to GPT; gdisk still works but is not required. The steps below use gdisk; fdisk can do the same.

1. Install gdisk (if not already installed):

   sudo dnf install gdisk -y

2. Start gdisk in interactive mode:

   sudo gdisk /dev/sdb

3. Create a new GPT partition:

   Command (? for help): n
   Partition number (1-128, default 1): 1
   First sector (34-209715166, default = 2048): Press Enter
   Last sector ... (default = 209715166): +50G
   Current type is 8300 (Linux filesystem)
   Hex code or GUID (L to show codes, Enter = 8300): Press Enter

4. View the partition layout:

   Command (? for help): p

5. Write and exit:

   Command (? for help): w

gdisk vs. fdisk Comparison

Feature	fdisk (MBR)	gdisk (GPT)
Maximum disk capacity	2TB	9.4ZB
Maximum number of partitions	4 primary	128 partitions
Partition table backup	None	One copy at each end of the disk
Use case	Legacy system compatibility	Recommended for modern servers

Caution

gdisk will convert an MBR disk to GPT format. If the disk already has MBR partitions containing data, be sure to back up first.

Notifying the Kernel of Partition Table Updates: partprobe

After creating or modifying partitions, if the system does not automatically recognize the new partition table, use partprobe to notify the kernel to re-read the partition information:

sudo partprobe /dev/sdb

Verify that the kernel has recognized the new partitions:

lsblk /dev/sdb
cat /proc/partitions | grep sdb

Note

If partprobe does not work, try partx -u /dev/sdb or reboot the system if necessary.

Creating Filesystems: mkfs

After creating partitions, they need to be formatted before use.

Creating an XFS Filesystem (RHEL/CentOS default)

sudo mkfs.xfs /dev/sdb1

Force overwrite an existing filesystem:

sudo mkfs.xfs -f /dev/sdb1

Creating an EXT4 Filesystem

sudo mkfs.ext4 /dev/sdb1

Creating a Filesystem with a Label

# XFS with label
sudo mkfs.xfs -L mydata /dev/sdb1

# EXT4 with label
sudo mkfs.ext4 -L mydata /dev/sdb1

Creating a Swap Partition

sudo mkswap /dev/sdb2
sudo swapon /dev/sdb2

Mounting and Unmounting

Temporary Mount

Create a mount point and mount the partition:

sudo mkdir -p /mnt/data
sudo mount /dev/sdb1 /mnt/data

Mount using UUID (recommended):

sudo mount UUID="a1b2c3d4-e5f6-7890-abcd-ef1234567890" /mnt/data

View current mount status:

mount | grep sdb
df -hT /mnt/data

Unmounting a Filesystem

sudo umount /mnt/data

If the device is reported as busy, find the processes using it:

sudo lsof +D /mnt/data
sudo fuser -mv /mnt/data

Lazy unmount (automatically unmounts after all users release it):

sudo umount -l /mnt/data

Caution

Before running umount, make sure no processes are accessing the mount point. Although umount -l (lazy unmount) will not report an error immediately, it still waits for all references to be released.

Persistent Mounting: /etc/fstab

Temporary mounts are lost after a system reboot. To mount automatically at boot, edit /etc/fstab.

Getting the Partition UUID

sudo blkid /dev/sdb1

Example output:

/dev/sdb1: UUID="a1b2c3d4-e5f6-7890-abcd-ef1234567890" TYPE="xfs"

Editing /etc/fstab

# Always back up first
sudo cp /etc/fstab /etc/fstab.bak
sudo vi /etc/fstab

Add the following line:

UUID=a1b2c3d4-e5f6-7890-abcd-ef1234567890  /mnt/data  xfs  defaults  0  0

fstab Field Descriptions

Field	Description	Example
Device identifier	UUID or device path	UUID=xxxx
Mount point	Target directory for mounting	/mnt/data
Filesystem type	xfs, ext4, swap, etc.	xfs
Mount options	Comma-separated list of options	defaults
dump backup	0 means do not use dump backup	0
fsck check order	0 = no check, 1 = root partition, 2 = other	0

Common Mount Options

Option	Description
defaults	Equivalent to rw,suid,dev,exec,auto,nouser,async
noexec	Prevent execution of binaries
nosuid	Prevent SUID/SGID bits from taking effect
nodev	Prevent interpretation of character or block devices
ro	Mount as read-only
noatime	Do not update access times; improves I/O performance
nofail	Do not block system boot if the device is absent

Verifying fstab Configuration

Test all fstab entries without rebooting:

sudo mount -a

Check whether the mount was successful:

df -hT /mnt/data

Danger

An incorrect /etc/fstab configuration can prevent the system from booting normally. Always back up the original file before making changes, and verify immediately with mount -a afterward. It is recommended to use the nofail option for non-critical storage.

Best Practices for Using UUIDs

Device names (such as /dev/sdb1) can change when hard drives are added or removed. UUIDs are unique partition identifiers that are not affected by device ordering.

Multiple Ways to View UUIDs

# Method 1: blkid
sudo blkid

# Method 2: lsblk -f
lsblk -f

# Method 3: View by-uuid symlinks
ls -l /dev/disk/by-uuid/

Recommended fstab Format

# Recommended: Use UUID with nofail to prevent boot failures
UUID=a1b2c3d4-e5f6-7890-abcd-ef1234567890  /mnt/data  xfs  defaults,nofail  0  0

# Not recommended: Using device names, which may change
/dev/sdb1  /mnt/data  xfs  defaults  0  0

Hands-On: Partitioning a New Disk and Configuring Auto-Mount

The following demonstrates the complete process of creating a GPT partition on a brand new 100G disk /dev/sdb, formatting it, and configuring automatic mounting.

1. Confirm the disk status:

   lsblk
   sudo fdisk -l /dev/sdb

2. Create a GPT partition with gdisk:

   sudo gdisk /dev/sdb
   # Interactive steps: n -> 1 -> Enter -> +80G -> Enter -> w -> Y

3. Notify the kernel to recognize the new partition:

   sudo partprobe /dev/sdb
   lsblk /dev/sdb

4. Format as an XFS filesystem:

   sudo mkfs.xfs /dev/sdb1

5. Create a mount point and temporarily mount:

   sudo mkdir -p /data
   sudo mount /dev/sdb1 /data
   df -hT /data

6. Get the UUID and configure fstab:

   # Get the UUID
   UUID=$(sudo blkid -s UUID -o value /dev/sdb1)
   echo "UUID: $UUID"
   
   # Back up fstab
   sudo cp /etc/fstab /etc/fstab.bak
   
   # Write to fstab
   echo "UUID=$UUID  /data  xfs  defaults,nofail  0  0" | sudo tee -a /etc/fstab

7. Verify the auto-mount configuration:

   sudo umount /data
   sudo mount -a
   df -hT /data

With this, the disk partitioning and mount configuration are complete. The /data directory will be automatically mounted after a system reboot.