What is swap and why resize it?

Swap is a designated portion of your hard drive that acts as an extension of your system's RAM. When your physical memory (RAM) fills up, the operating system moves less-used memory pages to swap space, freeing up physical RAM for active processes. Essentially, swap provides a safety net to prevent out-of-memory errors, albeit at the cost of slower performance compared to RAM.

Common reasons to resize swap:

• Insufficient swap space: If your system is running out of memory during peak usage

• Over-allocation: If you've allocated too much disk space to swap that could be better used elsewhere

• Performance tuning: Adjusting swap to match your system's workload patterns

• Preventing OOM (Out Of Memory) kills: Particularly important for database servers or memory-intensive applications

Recommended swap sizes:

Prerequisites

Before modifying your swap configuration, ensure you have:

• Root or sudo privileges: All commands in this guide require elevated permissions

• Sufficient free disk space: Verify available space with df -h

• Backup of critical data: System-level changes always carry some risk

• Understanding of current swap usage: Check with these commands:

# Show current swap files/partitions and their sizes
sudo swapon --show

# View memory and swap usage
free -h

# Check if swap is being actively used
vmstat 1 5

Step-by-Step process to resize swap

1. Prepare your system

Before disabling swap, check if your system has enough free RAM to handle the current swap usage:

# Check current memory usage
free -h

If swap is heavily used, consider closing memory-intensive applications first or performing this operation during a maintenance window.

2. Turn off all running swap processes

sudo swapoff -a

This command disables all swap spaces defined in /etc/fstab and currently active. It may take some time to complete as the system moves data from swap back to RAM.

3. Resize the swap file

For this example, we'll create a 4GB swap file (adjust the size to your needs):

# Remove the old swap file if it exists
sudo rm /swapfile

# Create a new swap file with desired size (4G in this example)
sudo fallocate -l 4G /swapfile

# Alternative method if fallocate fails:
# sudo dd if=/dev/zero of=/swapfile bs=1G count=4

Note: Replace 4G with your desired swap size (e.g., 2G for 2GB, 8G for 8GB).

4. Set proper permissions and ownership

Swap files should be readable and writable only by root for security:

# Set correct permissions (only root can read/write)
sudo chmod 600 /swapfile

# Verify permissions
ls -lh /swapfile

5. Set up the swap area

# Format the file as swap
sudo mkswap /swapfile

You should see output confirming the swap file creation.

6. Activate the swap file

# Enable the swap file
sudo swapon /swapfile

# Verify the swap is active
sudo swapon --show
free -h

7. Make the changes persistent across reboots

To ensure your swap file is automatically activated after system restart, you need to update the /etc/fstab file:

# First, backup the fstab file
sudo cp /etc/fstab /etc/fstab.bak

# Check if swap entry already exists
grep -q "swapfile" /etc/fstab || echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Optimizing swap performance

Adjusting swappiness

Swappiness is a kernel parameter that influences how aggressively the system uses swap space. Values range from 0 to 100:

• Lower values (e.g., 10): The system will try to avoid swapping as much as possible

• Higher values (e.g., 60): The system will swap out memory pages more aggressively

# Check current swappiness value
cat /proc/sys/vm/swappiness

# Temporarily change swappiness (until reboot)
sudo sysctl vm.swappiness=10

# Make the change permanent (add to /etc/sysctl.conf)
echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf

Adjusting cache pressure

Cache pressure determines how aggressively the kernel reclaims memory used for caching filesystem data:

# Check current cache pressure
cat /proc/sys/vm/vfs_cache_pressure

# Set a more balanced value (100 is default)
sudo sysctl vm.vfs_cache_pressure=50

# Make permanent
echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.conf

Troubleshooting common issues

Swap file not activating after reboot

If your swap doesn't activate after reboot:

1. Check /etc/fstab for correct syntax:

   cat /etc/fstab
   

2. Manually try to enable the swap:

   sudo swapon /swapfile
   

3. Look for errors in system logs:

   sudo dmesg | grep -i swap
   sudo journalctl -xe
   

"Operation not permitted" error

If you see this error when trying to create or enable swap:

1. Check if you're using a filesystem that supports swap files (most do, but some like NFS don't)

2. Verify you have proper permissions and are using sudo

Performance issues

If your system becomes sluggish after configuring swap:

1. Consider reducing swappiness to a lower value (10-30)

2. Check if your swap is on an SSD for better performance

3. Monitor swap usage with htop or free -h to see if your system is swap-thrashing

Performance considerations

• SSD vs HDD: Swap on an SSD will be significantly faster than on a traditional hard drive

• Separate partition: For heavy swap usage, consider using a dedicated partition instead of a file

• Multiple swap files: For very large systems, multiple swap files on different physical disks can improve performance

Monitoring swap usage

To effectively monitor swap usage over time:

# Install sysstat package for monitoring tools
sudo apt install sysstat

# Monitor swap usage every 5 seconds
vmstat 5

# Get detailed memory and swap statistics
free -h

# For graphical monitoring, consider installing and using htop
sudo apt install htop
htop

Conclusion

Properly configured swap space is an important part of system memory management in Ubuntu. While it's not a replacement for physical RAM, it provides a safety buffer against memory exhaustion and helps prevent application crashes during periods of high memory demand.

For production environments on Ploi, regularly monitor your swap usage patterns to determine if you need to adjust the swap size or consider upgrading your server's RAM for better performance.