Server Management·3 min lezen

Swap File on Linux

How to create and configure a swap file to prevent crashes from out-of-memory on Linux VPS

Swap is disk space used as virtual memory when RAM is full. On VPS with little RAM (1-2 GB) it is essential to prevent critical processes from crashing (OOM Killer).

Swap on SSD is much slower than real RAM. It doesn't replace RAM, but prevents sudden crashes. For intensive workloads, the correct solution is to upgrade your plan.

Check current swap

bash

# Swap status
free -h
swapon --show

# Swap usage per process
cat /proc/meminfo | grep -i swap

Create a swap file

Recommended size

RAM available	Recommended swap
512 MB - 1 GB	1-2 GB
2 GB	2 GB
4 GB	2-4 GB
8 GB+	2 GB (rarely need more)

Procedure

bash

# 1. Create the file (e.g. 2 GB)
sudo fallocate -l 2G /swapfile

# If fallocate is not available:
# sudo dd if=/dev/zero of=/swapfile bs=1M count=2048

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

# 3. Format as swap
sudo mkswap /swapfile

# 4. Activate swap
sudo swapon /swapfile

# 5. Verify
free -h
swapon --show

Make swap permanent

Add it to /etc/fstab to activate it on every reboot:

bash

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

# Verify /etc/fstab is correct
cat /etc/fstab | grep swap

Adjust swappiness

vm.swappiness controls how aggressively the kernel uses swap (0-100):

bash

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

# Set to 10 (more conservative)
sudo sysctl vm.swappiness=10

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

0 = use swap only if RAM is completely full
10 = recommended for VPS (rarely uses swap)
60 = default Ubuntu (too aggressive for server)

Adjust cache pressure

bash

# Reduces kernel's tendency to clear filesystem cache
sudo sysctl vm.vfs_cache_pressure=50
echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.conf

Resize swap

bash

# Disable current swap
sudo swapoff /swapfile

# Resize (e.g. to 4 GB)
sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

# Verify
free -h

Remove swap

bash

sudo swapoff /swapfile
sudo rm /swapfile

# Remove the line from /etc/fstab
sudo sed -i '/swapfile/d' /etc/fstab

Swap on partition (alternative)

If you have an additional disk or free partition:

bash

# Format the partition as swap
sudo mkswap /dev/sdb1

# Activate
sudo swapon /dev/sdb1

# Permanent in /etc/fstab
echo '/dev/sdb1 none swap sw 0 0' | sudo tee -a /etc/fstab

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