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Linux Mounts and /etc/fstab Explained: UUIDs, findmnt, Disk Usage and Troubleshooting

Learn Linux mounts and /etc/fstab with UUIDs, findmnt, lsblk, mount, umount, persistent mounts, disk and inode usage, deleted-open files, read-only filesystems and troubleshooting.

Melochip Editorial

@melochip-editorial

Published Sep 15, 2026 Updated Sep 15, 2026 4 min read
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Linux mounts and /etc/fstab graphic showing UUIDs, findmnt, and filesystem commands
Linux mounts and /etc/fstab graphic showing UUIDs, findmnt, and filesystem commands
Contents

Linux attaches filesystems into one directory tree starting at /. Instead of drive letters, storage devices, network shares, and bind mounts appear at directory paths such as /data, /mnt/share, or /srv/app-data.

How Linux Mounts Storage

text
Disk /dev/sdb1
    |
    v
Mounted at /data
    |
    v
Applications read and write /data/...

Inspect Disks, Filesystems and Mount Points

Terminal
$ lsblk -f findmnt df -hT blkid mount | column -t
  • lsblk -f shows block devices, filesystems, labels, UUIDs, and mount points.
  • findmnt shows the active mount tree.
  • df -hT shows filesystem type and human-readable capacity.
  • blkid prints filesystem identifiers such as UUID and TYPE.
  • mount lists mounted filesystems and options.

Create a Temporary Mount

Terminal
$ sudo mkdir -p /mnt/data sudo mount /dev/sdb1 /mnt/data findmnt /mnt/data df -hT /mnt/data
Terminal
$ sudo umount /mnt/data

Why UUIDs Are Better Than /dev/sdb1

Terminal
$ sudo blkid /dev/sdb1 lsblk -f
Expected output
/dev/sdb1: UUID="4ab1-example-uuid" TYPE="ext4"

Persistent Mounts with /etc/fstab

Terminal
$ sudo mkdir -p /data
Terminal
$ sudo cp /etc/fstab /etc/fstab.backup
text
UUID=4ab1-example-uuid /data ext4 defaults,nofail 0 2
Terminal
$ sudo mount -a findmnt /data findmnt --verify

Common /etc/fstab Fields

  • UUID/device: the filesystem identifier or device path to mount.
  • Mount point: the directory where the filesystem appears.
  • Filesystem type: ext4, xfs, nfs, vfat, or another supported type.
  • Options: mount behavior such as defaults, nofail, ro, or noexec.
  • Dump: legacy backup field, commonly 0.
  • fsck order: filesystem check order, commonly 1 for root, 2 for local data filesystems, and 0 to skip.

Useful Mount Options

  • defaults
  • nofail
  • ro
  • rw
  • nodev
  • nosuid
  • noexec

Unmount: Target Is Busy

Disk Full vs Inode Full

Terminal
$ df -h df -i
Terminal
$ sudo du -xhd1 / | sort -h sudo du -xhd1 /var | sort -h

When df and du Disagree

If df reports used space that du cannot find, a deleted file may still be open by a running process. The directory entry is gone, but the space is not released until the process closes the file.

Terminal
$ sudo lsof +L1 sudo lsof | grep '(deleted)'

Read-Only Filesystem Problems

Terminal
$ findmnt -o TARGET,SOURCE,FSTYPE,OPTIONS dmesg | tail -100 journalctl -k -b

Persistent Mount Fails During Boot

  • Wrong UUID
  • Wrong filesystem type
  • Missing device
  • Invalid option
  • Network storage unavailable
  • Mount-point typo
Terminal
$ cat /etc/fstab sudo blkid lsblk -f sudo mount -a findmnt --verify journalctl -xb

NFS Mount Example

Terminal
$ sudo apt install -y nfs-common sudo mkdir -p /mnt/share sudo mount -t nfs nfs-server:/export/share /mnt/share findmnt /mnt/share
text
nfs-server:/export/share /mnt/share nfs defaults,_netdev 0 0

Bind Mounts

Terminal
$ sudo mkdir -p /srv/app-data sudo mount --bind /data/app /srv/app-data findmnt /srv/app-data

Common Troubleshooting

Best Practices

  • Use UUIDs
  • Back up /etc/fstab
  • Run mount -a before rebooting
  • Monitor blocks/inodes
  • Use nofail deliberately
  • Plan storage growth
  • Back up data before repair/resize/formatting

Conclusion

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