Mounting and fstab

Chapter 14: Mounting and fstab

Overview

This chapter covers mounting file systems, configuring automatic mounting with /etc/fstab, and systemd mount units for modern Linux systems.

Why This Matters in DevOps/SRE

Proper mounting configuration ensures systems boot correctly, data persists, and storage is available. Misconfigured fstab is a common cause of boot failures and downtime.

┌─────────────────────────────────────────────────────────────────────────────┐
│                    MOUNTING IN DEVOPS                                       │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│  ┌─────────────────────────────────────────────────────────────────────┐    │
│  │                    COMMON INCIDENT SCENARIOS                          │    │
│  │                                                                       │    │
│  │   • Boot failure - Wrong fstab entry                                 │    │
│  │   • Mount timeout - Network storage unreachable                       │    │
│  │   • Read-only filesystem - Options misconfigured                     │    │
│  │   • Mount order - Dependencies not set                                │    │
│  │                                                                       │    │
│  └─────────────────────────────────────────────────────────────────────┘    │
│                                                                             │
│  ┌─────────────────────────────────────────────────────────────────────┐    │
│  │                    PRODUCTION MOUNT OPTIONS                            │    │
│  │                                                                       │    │
│  │   Data Mounts:                                                        │    │
│  │   • noatime,nodiratime - Better performance                          │    │
│  │   • barrier=1 - Data integrity                                       │    │
│  │   • defaults - Standard options                                      │    │
│  │                                                                       │    │
│  │   Network Mounts:                                                     │    │
│  │   • _netdev - Wait for network                                       │    │
│  │   • x-systemd.automount - On-demand mounting                         │    │
│  │   • soft,intr - Graceful failure                                      │    │
│  │                                                                       │    │
│  └─────────────────────────────────────────────────────────────────────┘    │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

14.1 Mount Basics

mount Command

# Mount basic
sudo mount /dev/sdb1 /mnt/data

# Mount with options
sudo mount -o rw,noexec /dev/sdb1 /mnt/data

# Mount read-only
sudo mount -o ro /dev/sdb1 /mnt/data

# Remount
sudo mount -o remount,rw /

# Unmount
sudo umount /mnt/data
sudo umount /dev/sdb1

# Lazy unmount (force)
sudo umount -l /mnt/data

# Check mounts
mount
findmnt

Common Mount Options

Option	Description
rw	Read-write
ro	Read-only
suid	Allow setuid
nosuid	Disallow setuid
exec	Allow executables
noexec	Disallow executables
auto	Mount automatically
noauto	No auto mount
user	Allow user mount
nouser	Only root can mount
defaults	rw,suid,dev,exec,auto,nouser,async
nofail	Don’t fail if device missing

14.2 /etc/fstab

fstab Format

<device> <mount point> <type> <options> <dump> <fsck>

Example fstab

# Device                          Mount Point    Type   Options        Dump  Pass
UUID=abc123                      /              ext4   defaults        1     1
UUID=def456                      /boot          ext4   defaults        0     2
UUID=ghi789                      /home          ext4   defaults        0     2
UUID=jkl012                      none           swap   sw              0     0
/dev/sdb1                        /mnt/data      ext4   defaults        0     2

Fields Explained

Field	Description
Device	Device path, UUID, or LABEL
Mount Point	Target directory
Type	Filesystem type (ext4, xfs, nfs, etc.)
Options	Mount options
Dump	Backup (0/1)
fsck	Order (0=skip, 1=root, 2=other)

14.3 UUID and Labels

Using UUID

# Find UUID
sudo blkid
ls -l /dev/disk/by-uuid/

# Use in fstab
UUID=abc123-def456-ghi789 / ext4 defaults 0 1

Using Labels

# Label filesystem
sudo e2label /dev/sdb1 mydata
sudo xfs_admin -L mydata /dev/sdb1

# Use in fstab
LABEL=mydata /mnt/data ext4 defaults 0 2

14.4 Systemd Mount Units

Mount Units

[Unit]
Description=Data Mount
After=network.target

[Mount]
What=/dev/sdb1
Where=/mnt/data
Type=ext4
Options=defaults

[Install]
WantedBy=multi-user.target

Enable Mount

sudo systemctl daemon-reload
sudo systemctl enable mnt-data.mount
sudo systemctl start mnt-data.mount

# Check status
systemctl status mnt-data.mount

Automount Units

[Unit]
Description=Data Automount

[Automount]
Where=/mnt/data

[Install]
WantedBy=multi-user.target

14.5 Common Mount Examples

NFS

# Mount NFS
sudo mount -t nfs server:/share /mnt/nfs

# With options
sudo mount -t nfs -o rw,hard,intr server:/share /mnt/nfs

# fstab entry
server:/share /mnt/nfs nfs defaults 0 0

CIFS/SMB

# Mount SMB
sudo mount -t cifs //server/share /mnt/share -o user=username

# With password
sudo mount -t cifs //server/share /mnt/share -o user=username,password=pass

# fstab entry
//server/share /mnt/share cifs credentials=/etc/samba/credentials 0 0

ISO Image

# Mount ISO
sudo mount -o loop,ro image.iso /mnt/iso

# fstab entry
/path/to/image.iso /mnt/iso iso9660 ro,loop 0 0

tmpfs (RAM Disk)

# Mount tmpfs
sudo mount -t tmpfs -o size=2G tmpfs /mnt/ramdisk

# fstab entry
tmpfs /mnt/ramdisk tmpfs size=2G 0 0

14.6 Interview Questions

Q: What is the difference between mount and fstab?
A: mount: command to mount immediately; fstab: automatic mount at boot

Q: What does the 'nofail' option do?
A: Don't fail if device doesn't exist (useful for optional mounts)

Q: What is the fsck field in fstab?
A: Order of filesystem check at boot (0=skip, 1=root, 2=other)

Q: How do you find UUID of a device?
A: sudo blkid or ls -l /dev/disk/by-uuid/

Q: What is systemd automount?
A: Mount on first access (on-demand)

Common Mistakes & Anti-Patterns

1. Using Device Names Instead of UUIDs

WRONG:

/dev/sdb1    /data    ext4    defaults    0 2
# Device names can change!

CORRECT:

UUID=abc123-def456-ghi789    /data    ext4    defaults    0 2
# OR
/dev/disk/by-uuid/abc123-def456-ghi789    /data    ext4    defaults    0 2

Why: Device names (/dev/sdX) can change between boots, especially in cloud environments.

2. Noatime Missing for Performance

WRONG:

# Default options - includes atime updates
/dev/sdb1    /data    ext4    defaults    0 2
# Every read updates access time - unnecessary I/O

CORRECT:

# With performance options
/dev/sdb1    /data    ext4    defaults,noatime,nodiratime    0 2

Why: noatime eliminates unnecessary writes on every file read.

3. Network Mount Without _netdev

WRONG:

# NFS mount without network wait
server:/share    /mnt/nfs    nfs    defaults    0 0
# Boot hangs if network not ready!

CORRECT:

# Wait for network before mounting
server:/share    /mnt/nfs    nfs    defaults,_netdev,x-systemd.requires=network-online.target    0 0

Why: _netdev ensures the system waits for network before attempting mount.

4. Wrong fsck Order

WRONG:

# Root set to 2 (should be 1)
UUID=...    /        ext4    defaults    0 2
# Should be: 0 1

# Non-root set to 1 (slows boot)
UUID=...    /data    ext4    defaults    0 1
# Should be: 0 2

CORRECT:

# Root: 0 1
UUID=...    /        ext4    defaults    0 1

# Other: 0 2
UUID=...    /data    ext4    defaults    0 2

# Skip: 0 0
tmpfs       /tmp     tmpfs   defaults    0 0

Why: fsck order determines boot time and what’s checked.

5. No Dump/Pass Settings

WRONG:

# Missing last two fields
/dev/sdb1    /data    ext4    defaults

CORRECT:

# dump: 0=don't backup, 1=backup
# pass: 0=skip fsck, 1=root, 2=check after root
/dev/sdb1    /data    ext4    defaults    0 2

Why: Proper pass order ensures correct filesystem checking sequence.

Summary

mount: Command to mount filesystems
fstab: Automatic mount configuration
UUID: Preferred way to identify devices
systemd: Modern mount units

Next Chapter

Chapter 15: Disk Quotas and Limits

Last Updated: February 2026