How To Add Swap Space on Ubuntu 16.04

Check the System for Swap Information

Before we begin, we can check if the system already has some swap space available. It is possible to have multiple swap files or swap partitions, but generally one should be enough.

We can see if the system has any configured swap by typing:

sudo swapon --show

If you don't get back any output, this means your system does not have swap space available currently.

You can verify that there is no active swap using the free utility:

free -h

Output
total        used        free      shared  buff/cache   available
Mem:           488M         36M        104M        652K        348M        426M
Swap:            0B          0B          0B

As you can see in the "Swap" row of the output, no swap is active on the system.

Check Available Space on the Hard Drive Partition

The most common way of allocating space for swap is to use a separate partition devoted to the task. However, altering the partitioning scheme is not always possible. We can just as easily create a swap file that resides on an existing partition.

Before we do this, we should check the current disk usage by typing:

df -h

Output
Filesystem      Size  Used Avail Use% Mounted on
udev            238M     0  238M   0% /dev
tmpfs            49M  624K   49M   2% /run
/dev/vda1        20G  1.1G   18G   6% /
tmpfs           245M     0  245M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           245M     0  245M   0% /sys/fs/cgroup
tmpfs            49M     0   49M   0% /run/user/1001

The device under /dev is our disk in this case. We have plenty of space available in this example (only 1.1G used). Your usage will probably be different.

Although there are many opinions about the appropriate size of a swap space, it really depends on your personal preferences and your application requirements. Generally, an amount equal to or double the amount of RAM on your system is a good starting point. Another good rule of thumb is that anything over 4G of swap is probably unnecessary if you are just using it as a RAM fallback.

Create a Swap File

Now that we know our available hard drive space, we can go about creating a swap file within our filesystem. We will create a file of the swap size that we want called swapfile in our root (/) directory.

The best way of creating a swap file is with the fallocate program. This command creates a file of a preallocated size instantly.

Since the server in our example has 512MB of RAM, we will create a 1 Gigabyte file in this guide. Adjust this to meet the needs of your own server:

sudo fallocate -l 1G /swapfile

We can verify that the correct amount of space was reserved by typing:

ls -lh /swapfile

-rw-r--r-- 1 root root 1.0G Apr 25 11:14 /swapfile

Our file has been created with the correct amount of space set aside.

Enabling the Swap File

Now that we have a file of the correct size available, we need to actually turn this into swap space.

First, we need to lock down the permissions of the file so that only the users with root privileges can read the contents. This prevents normal users from being able to access the file, which would have significant security implications.

Make the file only accessible to root by typing:

sudo chmod 600 /swapfile

Verify the permissions change by typing:

ls -lh /swapfile

Output
-rw------- 1 root root 1.0G Apr 25 11:14 /swapfile

As you can see, only the root user has the read and write flags enabled.

We can now mark the file as swap space by typing:

sudo mkswap /swapfile

Output
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
no label, UUID=6e965805-2ab9-450f-aed6-577e74089dbf

After marking the file, we can enable the swap file, allowing our system to start utilizing it:

sudo swapon /swapfile

We can verify that the swap is available by typing:

sudo swapon --show

Output
NAME      TYPE  SIZE USED PRIO
/swapfile file 1024M   0B   -1

We can check the output of the free utility again to corroborate our findings:

free -h

Output
total        used        free      shared  buff/cache   available
Mem:           488M         37M         96M        652K        354M        425M
Swap:          1.0G          0B        1.0G

Our swap has been set up successfully and our operating system will begin to use it as necessary.

Make the Swap File Permanent

Our recent changes have enabled the swap file for the current session. However, if we reboot, the server will not retain the swap settings automatically. We can change this by adding the swap file to our /etc/fstab file.

Back up the /etc/fstab file in case anything goes wrong:

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

You can add the swap file information to the end of your /etc/fstab file by typing:

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