Enable Swap on Debian / Ubuntu

This article is translated from the Chinese original.

Recently I picked up a junky little NAT VPS. It kept running out of memory and shutting down. Enabling swap helps a bit, enough to keep it barely running.

What is swap?

Swap is like a backup tire for memory. When physical memory is not enough, the system moves older data out of RAM and lets it lie on disk instead. It is slow, painfully slow, but still better than having your programs die on the spot.

Check the swap partition

free -m

You can see that the swap partition is 0, which means it is not enabled.

Enable swap

A cleaner way is to create the swap file with fallocate:

fallocate -l 256MB /swapfile

A more old-school way:

dd if=/dev/zero of=/swapfile bs=1M count=256

In general, if your RAM is below 2 GB, setting swap to around 1.5 to 2 times the RAM is fine.

Set file permissions:

chmod 600 /swapfile

Activate the swap partition:

mkswap /swapfile
swapon /swapfile

After doing that, run free -m again and you should see that swap has been enabled.

Below is some cleanup work.

Start automatically on boot

echo "/swapfile swap swap defaults 0 0" >> /etc/fstab

Adjust the swappiness value

The default swappiness value is 60 (0 to 100). If it is too high, memory swapping happens too often and the CPU suffers.

You can check the value with:

cat /proc/sys/vm/swappiness

Set it a bit lower. This VPS is already a pile of recycled junk anyway.

Use this command:

echo "vm.swappiness=10" >> /etc/sysctl.conf

Then apply it:

sysctl -p

Enable it temporarily (will be lost after reboot)

sysctl vm.swappiness=10

Disable swap

Turn off the swap partition:

swapoff -v /swapfile

Edit fstab:

vim /etc/fstab

Delete this line:

/swapfile swap swap defaults 0 0

Delete the /swapfile file:

rm /swapfile

• Use swap wisely and your VPS will shut down less often; make it too large and your disk will wear out sooner.