How to Add Swap on Ubuntu: Size and Swappiness

This guide shows how to add swap on Ubuntu 24.04 LTS with a swap file, for a small VPS that runs short of memory during builds or database spikes, or where the OOM killer has stopped a process. You will check the server, size and create the file, make it permanent without risking the next boot and tune swappiness; the commands are the same on Debian 12.

What swap does and what it does not do

Swap is disk space that the kernel uses as an overflow for RAM. When active processes need memory and too little is free, inactive pages move to swap, which frees that RAM for other uses. Swap is slower than RAM, and Ubuntu's SwapFaq warns not to treat it as a complete replacement for physical memory. [1]

It helps in two cases:

• Idle memory. Pages that large programs use only at start-up, and memory reserved but not really used, can move to swap and leave the RAM to other programs and the disk cache. [1]
• Unexpected peaks. When a program goes wrong or a job needs far more memory than planned, swap gives you time to find out what happened. [1]

It does not help when the work itself needs more memory than the server has. Heavy swap use slows a system down, because swap competes with normal work for the same disk, and even solid-state storage is slower than RAM. [1] When memory runs out completely, the kernel's OOM killer kills a task it picks by heuristics, normally a memory-hungry process. [2]

Check memory, swap and disk space first

Three read-only commands show what the server has today:

free -h
sudo swapon --show
findmnt -D /

free -h shows used and free RAM and swap in human-readable units; its available column estimates the memory left for starting new applications without swapping. [3] swapon --show prints a table of swap areas [4]; empty output means no swap is active, and a swap file that already exists can be resized instead. findmnt -D / shows the type (FSTYPE) and free space (AVAIL) of the root filesystem. [5] The type decides how you create the file. On Debian 12, run the commands as root if your account has no sudo.

How big should the swap file be?

For a system with more than 1 GB of RAM that does not hibernate, Ubuntu's SwapFaq recommends at least the square root of the RAM size, rounded, and at most twice the RAM. Below 1 GB, use at least as much swap as RAM and at most twice that. [1] These rows come from the FAQ's table: [1]

Hibernation needs more because it writes the contents of RAM to swap before the machine powers off. [1] You do not normally hibernate a VPS, so the "No hibernation" column is your minimum. Unused swap only costs disk space, but if you would need more than twice your RAM, the FAQ says to add RAM instead. [1] The 2 GiB used below suits a 2 GB or 4 GB server.

How to add swap on Ubuntu with a swap file

Create the file, restrict it to root, format and enable it, then list it in /etc/fstab and test the entry. You do not need to change any partitions, because swap can live in an ordinary file. [1]

sudo fallocate -l 2G /swapfile

sudo dd if=/dev/zero of=/swapfile bs=1MiB count=$((2*1024))

sudo chmod 600 /swapfile

sudo mkswap /swapfile
sudo swapon /swapfile
sudo swapon --show
free -h

sudo cp /etc/fstab /etc/fstab.bak
echo '/swapfile none swap defaults 0 0' | sudo tee -a /etc/fstab

sudo findmnt --verify
sudo systemctl daemon-reload
sudo swapoff /swapfile
sudo swapon -a
sudo swapon --show

Swappiness: what it is and how to change it

Swappiness tells the kernel how costly swap IO is compared with filesystem IO when it frees memory. It runs from 0 to 200: at 100 both count as equal, lower values make swap look more costly and higher values make it look less costly. The default is 60. [2] The kernel documentation adds that filesystem IO under memory pressure tends to be more efficient than swap's random IO, that the best value depends on the workload, and that at 0 the kernel does not start swapping until free and file-backed pages drop below a zone's high watermark. [2]

Ubuntu's SwapFaq suggests 10 for desktops, but says the default of 60 is likely more suitable for Ubuntu server installations. [1] To check the value and try another one until the next reboot (10 is only the FAQ's example): [1]

cat /proc/sys/vm/swappiness
sudo sysctl vm.swappiness=10

To keep a value, put it in a .conf file in /etc/sysctl.d/, which systemd-sysctl reads at boot. The sysctl.d manual suggests a two-digit prefix from 60 to 90 for files in /etc; if two files set the same key, the one whose name sorts last wins, so check for others. [9]

echo 'vm.swappiness = 10' | sudo tee /etc/sysctl.d/60-swappiness.conf
grep -rs swappiness /etc/sysctl.conf /etc/sysctl.d/

Leave vm.vfs_cache_pressure, which sets how readily the kernel reclaims its directory and inode caches, at its default. At 0, those caches are never reclaimed under memory pressure, which can easily lead to out-of-memory conditions. [2]

How to resize or remove the swap file

To resize the file, switch it off, create it again at the new size and switch it back on; the fstab line stays the same as long as the path does. Switching swap off moves its contents back into RAM, so there must be room: the SwapFaq's script checks that free RAM is at least the used swap [1], and swapoff exits with status 2 when the system has too little memory to stop swapping. [4]

free -h
sudo swapoff /swapfile
sudo rm /swapfile
sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
sudo swapon --show

If you created the first file with dd, use dd again, with count=$((4*1024)) for 4 GiB.

To remove swap, switch it off, delete its line from /etc/fstab and delete the file. [1] In the editor, remove only the /swapfile line:

sudo swapoff /swapfile
sudo cp /etc/fstab /etc/fstab.bak
sudo nano /etc/fstab
sudo findmnt --verify
sudo systemctl daemon-reload
sudo rm /swapfile

When to add RAM instead of more swap

Add RAM when the server swaps all the time under normal load, when available memory stays near zero or when the OOM killer still stops processes. Watch the server while it is busy:

vmstat 5 12
free -h

vmstat 5 12 prints twelve reports, five seconds apart; ignore the first, which shows averages since the last reboot. si is memory swapped in from disk per second and so is memory swapped out to disk. [10] These signs point to more RAM, not a bigger swap file:

• Constant swapping. si and so stay above zero in most reports during normal work: the heavy swap use that causes performance problems. [1]
• No headroom. The available figure in free -h stays near zero. [3]
• Swap past the ceiling. You would need more than twice your RAM in swap. [1]
• OOM kills continue. By default, the kernel produces a task dump on each OOM kill that shows why the OOM killer ran and why it chose that task; look for it in the kernel log. [2]

For the other limits to watch, see our guides to the signs that your VPS needs more resources and common VPS performance bottlenecks.

StealthRDP plans in the USA and EU all use NVMe storage, but swap on NVMe is still storage, not memory. If the signs persist, upgrade to a plan with more RAM at any time from the client area, paying the prorated difference; the Linux VPS page compares the plans.

FAQ

How much swap should I add on Ubuntu?

Ubuntu's SwapFaq says a system with more than 1 GB of RAM that does not hibernate needs at least the rounded square root of its RAM in swap, and at most twice its RAM. For a 4 GB server, that means 2 GB to 8 GB. [1]

Is a swap file slower than a swap partition?

Ubuntu's SwapFaq says that with the 2.6 kernel, a swap file is just as fast as a swap partition, so you do not need to reinstall to get more swap. It notes that a file may not use contiguous disk blocks the way a partition does. [1]

What is the default swappiness on Ubuntu?

The default is 60, both in the kernel documentation [2] and in Ubuntu's SwapFaq. [1] The kernel accepts values from 0 to 200, and its documentation says the best value depends on the workload. [2]

Can I use fallocate to create a swap file?

You can, and Ubuntu's SwapFaq uses fallocate in its own example. [1] The swapon manual warns that a preallocated file may count as a file with holes on some filesystems, which swapon rejects, and calls dd from /dev/zero the most portable method. [4]

How do I turn off swap on Ubuntu?

Run swapoff on the swap file, remove its line from /etc/fstab and delete the file. [1] swapoff needs enough free RAM to take back what is in swap, and it exits with status 2 when the system has too little memory to stop swapping. [4]

Does adding swap stop the OOM killer?

Not on its own. Swap is not a complete replacement for physical memory [1], and when the system runs out of memory, the kernel's OOM killer still kills a task, normally a memory-hungry one. [2]