How to Configure Swap File on Raspberry Pi

Swap space is essential for Raspberry Pi systems with limited RAM. This guide shows you how to create and configure a swap file to improve system stability and performance.

What is Swap?

Swap is disk space used as virtual memory when your system’s RAM is full. When RAM usage is high, Linux moves inactive pages from RAM to swap, freeing up memory for active processes.

Why You Need Swap on Raspberry Pi

  • Prevent Out-of-Memory (OOM) Errors: Protects against crashes when RAM is exhausted
  • Run Memory-Intensive Applications: Allows compilation, image processing, or databases
  • Improve Multitasking: System remains responsive under memory pressure
  • Enable Hibernation: Required for suspend-to-disk (if configured)

Note: While swap helps, it’s much slower than RAM. Don’t rely on it for performance-critical operations.

Prerequisites

  • Raspberry Pi with Raspbian/Raspberry Pi OS
  • Root or sudo access
  • Free disk space (recommended: 1-2GB for swap)
  • SD card with wear leveling (swap causes more writes)

Step 1: Check Current Swap Status

First, verify if swap is already configured:

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sudo swapon --show

Expected output:

  • If swap exists: Shows swap file/partition details (name, size, usage)
  • If no swap: No output (empty)

You can also check with:

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free -h

Look at the “Swap” row. If it shows 0B, no swap is configured.

Step 2: Create Swap File

Create a 1GB swap file using fallocate:

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sudo fallocate -l 1G /swapfile

Command breakdown:

  • fallocate: Quickly allocates disk space
  • -l 1G: Size of the swap file (1 gigabyte)
  • /swapfile: Location and name of the swap file

Choosing swap size:

  • 512MB RAM or less: 1-2GB swap
  • 1GB RAM: 1GB swap
  • 2GB RAM: 512MB-1GB swap
  • 4GB+ RAM: 512MB or none

Alternative method (if fallocate doesn’t work):

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sudo dd if=/dev/zero of=/swapfile bs=1M count=1024

This creates a 1GB file (1024 blocks × 1MB each).

Step 3: Verify File Creation

Check that the swap file was created with the correct size:

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ls -lh /swapfile

Expected output:

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-rw-r--r-- 1 root root 1.0G Dec 29 17:30 /swapfile

The file should be exactly 1GB.

Step 4: Set Correct Permissions

Critical security step! Swap files must be readable only by root to prevent information leaks:

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sudo chmod 600 /swapfile

Verify the permissions changed:

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ls -lh /swapfile

Expected output:

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-rw------- 1 root root 1.0G Dec 29 17:30 /swapfile

Notice the permissions are now 600 (only root can read/write).

Why this matters: Swap may contain sensitive data from memory (passwords, encryption keys). Wrong permissions could expose this data to other users.

Step 5: Format as Swap

Set up the file as swap space:

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sudo mkswap /swapfile

Expected output:

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Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
no label, UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

This formats the file with the swap signature and metadata.

Step 6: Enable Swap

Activate the swap file:

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sudo swapon /swapfile

The swap is now active! Verify it:

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sudo swapon --show

Expected output:

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NAME      TYPE SIZE USED PRIO
/swapfile file   1G   0B   -2

Or check with:

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free -h

You should see swap space listed under the “Swap” row.

Step 7: Make Swap Permanent

The swap file will be lost on reboot unless you add it to /etc/fstab.

Back up fstab first:

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sudo cp /etc/fstab /etc/fstab.backup

Add the swap file to fstab:

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

Verify it was added:

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tail -1 /etc/fstab

Expected output:

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/swapfile none swap sw 0 0

Now swap will automatically activate on boot.

Step 8: Adjust Swappiness (Optional)

Swappiness controls how aggressively the kernel uses swap. Value range: 0-100.

Check current swappiness:

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cat /proc/sys/vm/swappiness

Default: Usually 60

Recommended for Raspberry Pi:

  • 10: Prefer RAM, use swap only when necessary (recommended for SD cards)
  • 60: Default balanced behavior
  • 100: Aggressively use swap

Set swappiness temporarily (lost on reboot):

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sudo sysctl vm.swappiness=10

Make it permanent:

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echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf

Why lower swappiness on Pi:

  • Reduces SD card wear (fewer write cycles)
  • Improves performance (RAM is much faster)
  • Only uses swap when truly needed

Step 9: Adjust Cache Pressure (Optional)

Cache pressure determines how aggressively the kernel reclaims inode and dentry caches.

Check current value:

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cat /proc/sys/vm/vfs_cache_pressure

Default: 100

Set a lower value to keep more file system cache in memory:

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sudo sysctl vm.vfs_cache_pressure=50

Make it permanent:

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

Verification

Test that everything is working correctly:

Check Swap Status

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# Show swap devices
sudo swapon --show

# Show memory and swap usage
free -h

# Show detailed memory info
cat /proc/meminfo | grep -i swap

Test Swap Under Load

Create artificial memory pressure to force swap usage:

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# Install stress tool
sudo apt-get install stress

# Run memory stress test (use 90% of RAM)
stress --vm 1 --vm-bytes $(awk '/MemAvailable/{printf "%d\n", $2 * 0.9;}' < /proc/meminfo)k --timeout 30s

Monitor swap usage during the test:

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watch -n 1 free -h

You should see the “Swap” usage increase.

Troubleshooting

Swap Not Activating on Boot

Check fstab entry:

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cat /etc/fstab | grep swap

Test fstab manually:

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sudo swapoff /swapfile
sudo swapon -a  # Activates all swap in fstab
sudo swapon --show

“fallocate: fallocate failed: Operation not supported”

Some filesystems don’t support fallocate. Use dd instead:

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sudo dd if=/dev/zero of=/swapfile bs=1M count=1024 status=progress

Swap File Too Large for SD Card

Reduce swap size:

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# Remove existing swap
sudo swapoff /swapfile
sudo rm /swapfile

# Create smaller swap (512MB)
sudo fallocate -l 512M /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

System Still Running Out of Memory

  • Increase swap size: Follow removal steps, then create larger swap
  • Reduce swappiness: Set to 10 or lower
  • Close unnecessary services: Free up RAM
  • Add more physical RAM: Consider upgrading Pi model

SD Card Wearing Out Quickly

Swap causes many write cycles. To reduce wear:

  1. Lower swappiness to 10
  2. Use USB storage for swap instead of SD card:
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    sudo fallocate -l 1G /media/usb/swapfile
    # Follow same steps but use USB path
  3. Monitor SD card health regularly

Managing Swap

Disable Swap Temporarily

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sudo swapoff /swapfile

Re-enable Swap

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sudo swapon /swapfile

Remove Swap Completely

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# Disable swap
sudo swapoff /swapfile

# Remove fstab entry
sudo sed -i '/\/swapfile/d' /etc/fstab

# Delete swap file
sudo rm /swapfile

Resize Swap

To change swap size, you must remove and recreate it:

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# Disable current swap
sudo swapoff /swapfile

# Remove old swap file
sudo rm /swapfile

# Create new swap with desired size (e.g., 2GB)
sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

# Verify
sudo swapon --show

Performance Monitoring

Monitor swap usage over time:

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# Real-time monitoring
watch -n 2 'free -h && echo && swapon --show'

# Check swap I/O
iostat -x 2

# View processes using swap
for file in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file; done | sort -k 2 -n -r | head -10

Alternative: Using zram Instead

For better performance and less SD card wear, consider zram (compressed RAM):

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# Install zram tools
sudo apt install zram-tools

# Configure zram
echo 'ALGO=lz4' | sudo tee -a /etc/default/zramswap
echo 'PERCENT=50' | sudo tee -a /etc/default/zramswap

# Start zram
sudo systemctl start zramswap

# Enable on boot
sudo systemctl enable zramswap

Zram compresses memory in RAM itself, avoiding slow disk I/O and SD card wear.

Quick Reference

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# Check swap status
sudo swapon --show
free -h

# Create 1GB swap
sudo fallocate -l 1G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

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

# Optimize for SD card
sudo sysctl vm.swappiness=10
echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf

# Disable swap
sudo swapoff /swapfile

# Enable swap
sudo swapon /swapfile