Swap on a VPS: when to add it and how

Swap is space on the disk where the system moves data out of RAM when memory runs low. You'll add it in a few minutes, and thanks to it the VPS won't crash during a short spike. We'll also show when swap doesn't help and you need more RAM.

What swap is and when it helps#

When RAM runs out and there's no swap, the Linux kernel starts the OOM killer (“out of memory”) and kills the process that takes up the most memory. Usually that's exactly your database, game server or application. In the log you'll then find Out of memory: Killed process.

With swap, the system first moves rarely used data to the disk and the process keeps running.

Checking the current state#

swapon --show
free -h

• swapon --show – lists active swap. If it prints nothing, the VPS has no swap (that's normal for a new VPS).
• free -h – shows RAM and swap usage in readable units (-h = human readable).

In the free -h output, watch the available column on the Mem line: how much memory applications really have available. The Swap line shows the swap size and usage.

How big a swap to choose#

Swap takes up disk space, so don't make it needlessly big.

Creating a swap file#

This applies to both Ubuntu 24.04 and Debian 13. As an example we'll create a 2 GB swap in the file /swapfile.

1. Create a file of the required size:

   fallocate -l 2G /swapfile

   • fallocate – quickly reserves disk space for a file.
   • -l 2G – the file size (2G = 2 GB; for 4 GB write 4G).
   • /swapfile – the path to the file.

2. Set the permissions so that only root can read the file. Swap can contain chunks of memory with passwords:

   chmod 600 /swapfile

   • 600 – read and write only for the owner (root), nothing for others.

3. Prepare the file as swap:

   mkswap /swapfile

   • mkswap – writes a swap header to the file. It prints Setting up swapspace version 1, size = 2 GiB.

4. Turn swap on:

   swapon /swapfile

   • swapon – starts using the swap right away, but only until the next restart.

5. Check the result with swapon --show. You'll see the line /swapfile file 2G.

Enabling it permanently through /etc/fstab#

To have swap turn on after a restart too, add it to the /etc/fstab file.

1. Back up the file:

   cp /etc/fstab /etc/fstab.bak

2. Add a swap line to the end of the file:

   echo '/swapfile none swap sw 0 0' >> /etc/fstab

   • echo '…' – prints the text in quotes.
   • >> – appends the text to the end of the file. Careful: a single > would overwrite the whole file.

   What each column of the line means:

   Column	Value	Meaning
   1. device	/swapfile	The swap file.
   2. mount point	none	Swap isn't mounted anywhere.
   3. type	swap	It's swap.
   4. options	sw	Default options for swap.
   5. dump	0	Don't back up with the old dump tool.
   6. check	0	Don't check at boot (swap is never checked).

3. Check that fstab is fine:

   findmnt --verify

   • findmnt --verify – checks the syntax of /etc/fstab. The result must be Success, no errors or warnings detected, or a summary with 0 parse errors, 0 errors. Warnings alone (warnings) about other lines won't stop the VPS from booting.

Swappiness: how eagerly the system swaps#

The vm.swappiness value (0–⁠200) says how soon the kernel starts moving data to swap. The default is 60. On a server a lower value is better, so that swap is used mainly as a safety net.

1. Find the current value:

   cat /proc/sys/vm/swappiness

2. Save the new value to your own file (Debian 13 no longer reads /etc/sysctl.conf; the /etc/sysctl.d/ folder works on both systems):

   echo 'vm.swappiness=10' > /etc/sysctl.d/99-swappiness.conf

3. Load the setting without a restart:

   sysctl --system

   • sysctl --system – loads all settings from the sysctl.d folders. In the output you'll see the line vm.swappiness = 10.

Verification#

• swapon --show shows /swapfile with the right size.
• free -h has a non-zero total value on the Swap line.
• After a VPS restart, swap is active even without running swapon by hand.

Removing swap#

1. Turn swap off: swapoff /swapfile. If swap is heavily used, it takes longer because the data moves back to RAM. If there isn't enough RAM for that, the command ends with an error and nothing happens.
2. Delete the line /swapfile none swap sw 0 0 from the /etc/fstab file (nano /etc/fstab).
3. Delete the file: rm /swapfile.

Troubleshooting#

fallocate: fallocate failed: No space left on device#

There isn't enough space on the disk. Check the free space with df -h / and choose a smaller swap, or extend the disk.

swapon: /swapfile: insecure permissions 0644, 0600 suggested.#

The file's permissions are too open. Swap turns on, but anyone on the VPS can read the file. Run chmod 600 /swapfile.

swapon: /swapfile: swapon failed: Device or resource busy#

Swap is already on. Check with swapon --show.

The VPS is slow even with swap and swap is always full#

The applications permanently need more memory than the VPS has. Find out what's using memory (htop, sort by the MEM% column) and either give the application less memory or add RAM in the Change plan tab.

Further reading#

• Ubuntu: SwapFaq (opens in a new window)
• swapon(8) manual page (opens in a new window)
• mkswap(8) manual page (opens in a new window)