How to add swap space on Ubuntu 18.04

How to add swap space on Ubuntu 18.04

We have all had it happen to us. You are in the middle of some server operations and suddenly you get an Out of memory error. A very frustrating error. But what if there is an easy solution for it?

In a typical computer we usually have two basic types of memory.

One being the random access memory (RAM). RAM is used to store data and programs while they are being actively used. If a program or data is not stored in RAM they can not be used. When you turn off your computer the data stored in RAM is lost. This is why we call RAM volatile memory.

The second type of memory, when talking about Linux systems, is swap space.

Swap space

Swap space fulfills one major function and that is substituting disk space for RAM when the actual RAM is filling up and needs more space.

Let us say you have a Computer with 8GB of RAM. Now you start opening many programs. If your RAM does not fill up all the way you are good to go. But what if you open so many programs that your RAM does fill up? This is where swap space comes into play. Without swap space you would need to free up your RAM by closing unnecessary programs until your computer starts responding again. What swap space will do is use pre-allocated disk space and allow the RAM to use that part as “extra” RAM.

How to create swap space on Ubuntu 18.04

Step 1 – Check if swap space already exists

Before we create swap space we should check if the server already has some swap space available. While it is possible to have multiple swap files or swap partitions, one should be enough.

To check if the system has any configured swap space execute the following command:

$ sudo swapon --show

If you do not see any output it means your system does not have any swap space configured. To completely verify this we can use the free command.

$ free -h

Output
              total        used        free      shared  buff/cache   available
Mem:            10G        2,3G        5,5G        270M        2,9G        7,9G
Swap:            0B          0B          0B

Our swap row says it currently has zero bytes allocated, which means no swap is active.

Step 2 – Creating the swap file

Before we being, let us check if we have enough disk space available to create a swap file.

$ df -h

Output

Filesystem      Size  Used Avail Use% Mounted on
udev            5,4G     0  5,4G   0% /dev
tmpfs           1,1G  1,4M  1,1G   1% /run
/dev/sda1       393G  302G   72G  81% /
tmpfs           5,4G  173M  5,3G   4% /dev/shm
tmpfs           5,0M  4,0K  5,0M   1% /run/lock
tmpfs           5,4G     0  5,4G   0% /sys/fs/cgroup
tmpfs           1,1G   28K  1,1G   1% /run/user/121
tmpfs           1,1G   24K  1,1G   1% /run/user/1000

We are looking at our main mount which is /. We still have plenty of space in our case, 72G. When deciding on the amount of swap space to allocate a good rule of thumb is, double the amount of RAM on your system. Another pretty good rule is that anything above 4G is probably unnecessary. For a nice overview see the table below.

Now, let us finally start creating the actual swap space. In this guide we will make a swap space of 2G in this guide.

$ sudo fallocate -l 2G /swapfile

Let us verify if the correct amount was reserved

$ ls -lh /swapfile

Output
-rw-r–r– 1 root root 2,0G sep 13 2018 /swapfile

Step 4 Enabling our swap file

In the last step we have reserved space for our swap file, now we need to turn this into swap space.

First, since Linux is all about permissions, we will need to change the permissions of the file we made so that only root users can read it. If we do not execute this step we could have pretty severe security risks.

$ sudo chmod 600 /swapfile

$ ls -lh /swapfile

Output
-rw------- 1 root root 2,0G sep 13  2018 /swapfile

Next, we can tell our file that it is a swap file

$ sudo mkswap /swapfile

Now let us enable our newly created swapfile

$ sudo swapon /swapfile

Check whether the swap is available

$ sudo swapon --show

Output
NAME      TYPE  SIZE USED PRIO
/swapfile file 2048M   0B   -2

Step 5 Making a permanent swap file

Currently we have a swap file that only exists on this sessions. Which means that if we were to turn off our system, our swap file will disappear.

We can make our swap file by adding it to the </etc/fstab/code> file.

$ sudo cp /etc/fstab /etc/fstab.bak

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