Understanding UUIDs

Understanding UUIDs

UUID (Universally Unique Identifier) is a 128-bit identifier assigned to file systems at creation time. Linux uses UUIDs to refer to storage devices in /etc/fstab, bootloader configuration, and systemd mount units. The critical reason to use UUIDs instead of device names like /dev/sda1: device names change when you add, remove, or rearrange disks, but a UUID is permanently tied to the file system.

Why UUIDs exist for storage

Problem with device names:
  Server with 2 disks:  /dev/sda (system) + /dev/sdb (data)
  You add a new disk → system now sees it as /dev/sdb (new disk)
  → Old data disk becomes /dev/sdc
  → /etc/fstab entry for /dev/sdb now mounts the WRONG disk!

Solution with UUIDs:
  /etc/fstab: UUID=abc-123 → always mounts the specific file system
  No matter what device name the OS assigns, the UUID is correct.

Finding UUIDs

# Method 1: blkid (most common)
sudo blkid

blkid output

/dev/sda1: UUID="a1b2c3d4-e5f6-7890-abcd-ef1234567890" TYPE="ext4" PARTUUID="..."
/dev/sda2: UUID="12345678-1234-1234-1234-123456789abc" TYPE="swap"
/dev/sdb1: UUID="fedcba98-7654-3210-fedc-ba9876543210" TYPE="xfs" LABEL="data"

# Method 2: ls of symlinks in /dev/disk/by-uuid/
ls -la /dev/disk/by-uuid/

# Method 3: lsblk
lsblk -f

# Get UUID for a specific device
sudo blkid /dev/sdb1

# Get UUID only (for scripting)
sudo blkid -s UUID -o value /dev/sdb1

Using UUIDs in fstab

# Get the UUID
UUID=$(sudo blkid -s UUID -o value /dev/sdb1)
echo $UUID

# Add to /etc/fstab
sudo nano /etc/fstab

/etc/fstab entry using UUID

# UUID format in fstab:
UUID=fedcba98-7654-3210-fedc-ba9876543210   /data   xfs   defaults,noatime   0   2

# For swap partition:
UUID=12345678-1234-1234-1234-123456789abc   none    swap  sw                 0   0

# Test the fstab entry
sudo mount -a
df -h /data

Labels as an alternative

Labels are human-readable alternatives to UUIDs. They are easier to remember but require discipline to keep unique across all disks. UUIDs are always unique (randomly generated), so they are safer.

# Set a label on an ext4 file system
sudo tune2fs -L backups /dev/sdb1

# Set a label on an XFS file system (must be unmounted)
sudo xfs_admin -L backups /dev/sdb1

# Set a label when creating a file system
sudo mkfs.ext4 -L backups /dev/sdb1
sudo mkfs.xfs -L backups /dev/sdb1

# Use label in fstab
# LABEL=backups   /backup   ext4   defaults   0   2

# List all labels
sudo blkid | grep LABEL

When UUIDs change

UUIDs are assigned at file system creation and do not change in normal operation. They change only when:

• You run mkfs to reformat the partition (completely new file system)
• You clone a disk with a tool that copies the file system (then both disks have the same UUID — a problem)
• You explicitly regenerate the UUID with tune2fs -U random /dev/sdb1

# Generate a new UUID for an ext4 file system (if cloning caused duplicate UUIDs)
sudo tune2fs -U random /dev/sdb1
sudo blkid /dev/sdb1    # Verify the new UUID

# For XFS (must be unmounted)
sudo xfs_admin -U generate /dev/sdb1

# After changing UUID, update /etc/fstab to use the new UUID

Conclusion

Always use UUIDs in /etc/fstab instead of device names. Device names (sda, sdb) are assigned by the kernel at boot based on device detection order — adding a disk can shift everything around and break mounts that relied on device names. Get the UUID with sudo blkid /dev/sdbX and use the full UUID string in fstab. If you clone a disk, regenerate the UUID on the clone with tune2fs -U random.