Linux: Setup Swap Memory

Swap memory acts as an extension of physical RAM, providing additional virtual memory when the system runs low on physical memory. This guide will walk you through the process of setting up swap memory on a Linux system.

Prerequisites

Root or sudo access to the Linux system
Sufficient disk space for the swap file
Basic knowledge of Linux commands

Check Current Swap Memory

Before creating new swap space, it’s important to check if swap is already configured on your system.

Check swap memory using htop:
```
htop
```
Note

Press F10 or q to exit htop.
Display current swap configuration:
```
swapon --show
```
This command shows all active swap devices and their sizes.
View memory and swap usage:
```
free -h
```

Turn Off Existing Swap

If you need to modify or replace existing swap, you must turn it off first.

Warning

This operation moves data from swap to main memory and may take several minutes depending on the amount of data in swap.

Disable all swap spaces:
```
sudo swapoff -a
```
Verify swap is disabled:
```
swapon --show
```
The command should return no output if swap is successfully disabled.

Create New Swap File

Now we’ll create a new swap file with the desired size.

Create a 5GB swap file:
```
sudo dd if=/dev/zero of=/swapfile bs=1G count=5
```
Note
- if=/dev/zero - Input file (source of zeros)
- of=/swapfile - Output file (swap file location)
- bs=1G - Block size (1 Gigabyte)
- count=5 - Number of blocks (5 GB total)
You can adjust the count parameter to change the swap size (e.g., count=16 for 16GB).
Verify the file was created:
```
ls -lh /swapfile
```

Configure Swap File Permissions

For security reasons, the swap file should only be accessible by the root user.

Set correct permissions:
```
sudo chmod 0600 /swapfile
```
Verify permissions:
```
ls -l /swapfile
```
The output should show: -rw------- (readable and writable only by root)

Initialize Swap Area

Set up the Linux swap area:
```
sudo mkswap /swapfile
```
This command prepares the file for use as swap space.
Enable the swap file:
```
sudo swapon /swapfile
```
Verify swap is active:
```
swapon --show
```
You should see your new swap file listed.
Check memory and swap status:
```
free -h
```

Make Swap Permanent

By default, the swap configuration will be lost after a reboot. To make it permanent:

Create a backup of the fstab file:
```
sudo cp /etc/fstab /etc/fstab.back
```
Important

Always backup /etc/fstab before modifying it. This file is critical for system boot.

Add swap entry to fstab:

echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Verify the entry was added:
```
tail -1 /etc/fstab
```
You should see: /swapfile none swap sw 0 0

Final Verification

Check swap status:

swapon --show

Expected output:

NAME      TYPE SIZE USED PRIO
/swapfile file   5G   0B   -2

View detailed memory information:

free -h

Expected output should show swap space available:

total        used        free      shared  buff/cache   available
Mem:           15Gi       2.1Gi        10Gi       234Mi       3.2Gi        12Gi
Swap:           5Gi          0B         5Gi

Monitor system with htop:
```
htop
```
You should see the swap bar at the top showing your new swap space.

Fig. 37 Successfully configured 5GB swap memory

Troubleshooting

Swap file is not persisting after reboot

Verify the entry in /etc/fstab is correct
Check for typos in the fstab entry
Ensure the swap file path is absolute (/swapfile)

Insufficient disk space error

Check available disk space: df -h
Choose a smaller swap size
Free up disk space before creating swap

Permission denied errors

Ensure you’re using sudo for all swap commands
Verify you have root access to the system

Additional Resources

Summary

You have successfully configured swap memory on your Linux system. The swap file will:

Provide additional virtual memory when physical RAM is full
Persist across system reboots
Improve system stability under memory pressure

Tip

For optimal performance, it’s recommended to have sufficient physical RAM rather than relying heavily on swap space. Swap should be used as a safety buffer, not as a primary memory solution.