In this post, I describe how to create a swap file and enable swap space.

I prefer not to dedicate an entire disk partition to swap. Using a file offers more flexibility.

Creating a file for swap

First of all, you need to create a file that will act as the swap space:

  • sudo fallocate -l 1G /swap

Where:

  • 1G - the size of the file.
  • /swap - the filename and location.

Next, set the correct permissions on the file to ensure only the root user can read it:

  • sudo chmod 600 /swap

To check the created file and its permissions, use ls:

  • ls -lh /swap

Enabling the swap file

To turn the file into swap space, use mkswap and swapon:

  1. sudo mkswap /swap
  2. sudo swapon /swap

To check the current swap status, use swapon and free:

  • swapon --show
  • free -h

Making swap persistent

The swap space created above is not persistent and will vanish after reboot. To prevent this, you need to modify the fstab file.

First, it’s a good practice to create a backup of the existing fstab file:

  • sudo cp /etc/fstab /etc/fstab.bak

Then, add the created swap file to the file system table (fstab). This is done by modifying the /etc/fstab file and appending a line that describes the swap file location and its properties.

Use sudo nano /etc/fstab to modify the file. Add the following line at the end of the file:

  • /swap none swap sw 0 0

Where:

  • /swap - the location of the swap file.

Modifying swappiness

The kernel parameter vm.swappiness controls how aggressively the Linux kernel swaps memory pages. The default value is usually 60, but I find that my servers sometimes become unresponsive during heavy compilation tasks with this setting. I prefer this setting to be 10 (which means 10%).

To change swappiness immediately (until the next reboot), use sysctl:

  • sudo sysctl vm.swappiness=10

To make this change persistent across reboots, you need to modify the sysctl.conf file:

  • sudo nano /etc/sysctl.conf
    • Add or modify: vm.swappiness=10

To check the current swappiness value, run cat /proc/sys/vm/swappiness.

Summary

  1. Create the file: sudo fallocate -l 1G /swap
  2. Set permissions: sudo chmod 600 /swap
  3. Initialize swap: sudo mkswap /swap
  4. Enable swap: sudo swapon /swap
  5. Backup fstab: sudo cp /etc/fstab /etc/fstab.bak
  6. Edit fstab: sudo nano /etc/fstab
    • Add: /swap none swap sw 0 0
  7. Set runtime swappiness: sudo sysctl vm.swappiness=10
  8. Set persistent swappiness: sudo nano /etc/sysctl.conf
    • Add: vm.swappiness=10
Author: Savelii Pototskii
Date:
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Categories: linux