Chapter 9: Persistent Mounts & Storage Operations

Understanding UUID

A filesystem UUID is a stable identifier for a filesystem. Prefer stable identifiers over kernel device names such as /dev/sdb1, which can change after hardware or bus reordering.

Standard Linux filesystem UUIDs use the form 8-4-4-4-12 hexadecimal digits, for example 67bc1234-45ab-4def-8123-456789abcdef.

sudo blkid
sudo lsblk -f
# Example blkid line:
# /dev/sdb1: UUID="67bc1234-45ab-4def-8123-456789abcdef" TYPE="ext4"

What is Mounting?

A mount point is the directory where a filesystem is attached — covered in Chapter 7 §7.

Temporary Mount & Unmount

sudo mkdir -p /data
sudo mount /dev/sdb1 /data
sudo mount UUID=67bc1234-45ab-4def-8123-456789abcdef /data
sudo umount /data

/etc/fstab Configuration Table

The /etc/fstab file defines filesystems that should automatically mount during boot. Copy the UUID from blkid or lsblk -f — do not invent a shortened value.

# Illustrative example only — replace with your real UUID from blkid
UUID=67bc1234-45ab-4def-8123-456789abcdef  /data  ext4  defaults  0  2

Common Mount Options

sudo findmnt --verify --verbose
sudo mount -a
findmnt /data
journalctl -b -u '*.mount' --no-pager | tail

NFS Client Mounts

NFS lets a Linux host mount a remote directory exported by an NFS server. On the client you need the server hostname (or IP), the export path, and a local mount point.

List exports advertised by a server (when showmount is available):

showmount -e nfs-server.example.com

Mount temporarily for testing:

sudo mkdir -p /mnt/share
sudo mount -t nfs nfs-server.example.com:/export/data /mnt/share
# Some sites prefer NFSv4:
# sudo mount -t nfs4 nfs-server.example.com:/export/data /mnt/share
df -h /mnt/share
sudo umount /mnt/share

Persist the mount in /etc/fstab. For data-bearing shares, prefer hard so the client keeps retrying instead of returning partial I/O errors:

nfs-server.example.com:/export/data  /mnt/share  nfs  defaults,_netdev,hard,timeo=600  0  0

Automount with autofs

When users only need a share occasionally, mounting everything in /etc/fstab can slow boots and leave stale mounts. autofs mounts on first access and unmounts after idle timeout.

The master map is usually /etc/auto.master. A typical indirect map mounts under a base directory (for example /misc) using a map file such as /etc/auto.misc:

# /etc/auto.master
/misc   /etc/auto.misc

# /etc/auto.misc (example — hard preferred for data)
data   -fstype=nfs,hard,timeo=600   nfs-server.example.com:/export/data

After editing maps, restart or reload according to your unit (systemctl restart autofs is the portable choice when reload is unsupported). Access /misc/data to trigger the mount; confirm with findmnt /misc/data. Prefer autofs for home directories and on-demand NFS; keep always-on critical shares in fstab.

Disk Usage Commands

Repair ext4 & XFS Filesystems

Use this as an operations checklist. Identify the filesystem type first, unmount (or boot to rescue for root), take a backup or snapshot when possible, then run the matching repair tool.

1. Confirm device and type: lsblk -f / blkid
2. Unmount the target (or use rescue mode for the root filesystem)
3. Backup or snapshot when possible
4. Repair: ext4 → sudo fsck -f /dev/sdb1; XFS → sudo xfs_repair /dev/sdb2 (never run XFS repair through generic fsck)
5. Remount and verify: mount, findmnt, dmesg / journalctl -b

Swap Administration

Swap provides virtual memory when RAM is under pressure. Enabling a plain empty file fails until it has been sized and initialized with mkswap.

# Inspect current swap
swapon --show
free -h

# Create a 2G swapfile (lab or approved maintenance window)
# Prefer dd for a portable, non-sparse file. Wrong of= destroys data — verify the path.
# fallocate can leave sparse/CoW-unsuitable files on some filesystems (e.g. Btrfs).
sudo dd if=/dev/zero of=/swapfile bs=1M count=2048 status=progress
# Alternative on supported filesystems: sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

# Persist in fstab (use the swapfile path or its UUID from blkid)
# /swapfile  none  swap  sw  0  0

sudo swapoff /swapfile   # disable when intentionally removing swap

Deleted but Open Files

A deleted file may still consume disk space if a running process keeps it open.

lsof | grep deleted

Storage Performance Monitoring

Knowledge Check (MCQs)

1. Which command repairs an unmounted ext4 filesystem?

   • a) mount -a
   • b) fsck
   • c) df -h
   • d) swapon

2. Which tool repairs an XFS filesystem?

   • a) fsck
   • b) mkfs.xfs
   • c) xfs_repair
   • d) blkid

3. Which file stores persistent mount configuration?

   • a) /etc/hosts
   • b) /etc/fstab
   • c) /etc/exports
   • d) /proc/mounts

4. Which fstab option delays NFS mounts until networking is up?

   • a) noatime
   • b) defaults
   • c) _netdev
   • d) soft

5. Which command lists NFS exports from a server?

   • a) mount -t nfs
   • b) showmount -e
   • c) findmnt
   • d) df -h

6. Which command shows filesystem usage?

   • a) du
   • b) lsof
   • c) df -h
   • d) iotop

7. Which command shows directory size?

   • a) df
   • b) du
   • c) blkid
   • d) umount

8. Which command helps find deleted files still held open by a process?

   • a) lsof
   • b) fsck
   • c) swapon
   • d) mount

9. Which command enables swap?

   • a) swapoff
   • b) swapon
   • c) free -h
   • d) iostat

10. Which tool monitors disk I/O statistics?

    • a) hostname
    • b) iostat
    • c) chsh
    • d) echo

11. Why prefer UUID in fstab?

    • a) Faster boots than device names
    • b) Stable device identification across renames
    • c) Required for NFS only
    • d) Disables fsck automatically

12. Should fsck run on a mounted ext4 filesystem?

    • a) Yes, for online repair
    • b) Only with -f
    • c) No — unmount first
    • d) Only on XFS

Commands Covered in This Chapter

• blkid — show UUID and filesystem type
• mount / umount — mount or unmount filesystems
• mount -a / findmnt — apply and inspect fstab mounts
• showmount — list NFS exports from a server
• systemctl restart autofs — apply automount map changes (use reload only if the unit supports it)
• fsck / xfs_repair — check and repair filesystems
• df / du — report filesystem and directory usage
• swapon / swapoff — enable or disable swap
• lsof — list open files (including deleted ones)
• iostat / iotop / sar — monitor disk I/O