78.1 Swap File

20211210

If you are running out of swap memory you can add more by creating a swap file on your storage device (hdd or ssd). Swapping to a file is a little less efficient than swapping to a dedicated swap partition as the kernel needs to access the new swap file through the file system. But since this swap file is only called upon when you’ve run out of RAM and then swap memory on a dedicated partition, the additional performance hit is not much compared to that of swapping itself.

Have a look at what memory you have available using free:

$ free
               total        used        free      shared  buff/cache   available
Mem:        16028508     3987252     3494292     1043004     8546964    10656712
Swap:        1003516      467112      536404

Here we have 16GB memory and 1GB of swap of which something like 467MB is being used.

The swapon command can also show the swap memory utilisation:

$ swapon --show
NAME      TYPE      SIZE   USED PRIO
/dev/dm-2 partition 980M 456.2M   -2

Here we have almost 1GB of swap of which 456MB is being used.

To add, for example 4GB, to the swap space create a new file of that size and tell the operating system to use this as swap.

First, check the amount of storage space available:

$ df -h

Filesystem                 Size  Used Avail Use% Mounted on
tmpfs                      1.6G  3.2M  1.6G   1% /run
/dev/mapper/vgubuntu-root  467G  417G   26G  95% /
...

We’ll create the new swap file in root (/) using fallocate and then ensure it is not generally available to users to view:

$ sudo fallocate -l 4G /swapfile
$ sudo chmod 600 /swapfile

To set up the new file as a special swapfile we use mkswap:

$ sudo mkswap /swapfile

Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
no label, UUID=a1a0d080-e4bd-4cd8-ab09-0bb9c4eaf9e0

Now tell the operating system to use the file for swap:

sudo swapon /swapfile

Check it is recognised:

$ sudo swapon --show

NAME      TYPE      SIZE   USED PRIO
/dev/dm-2 partition 980M 456.2M   -2
/swapfile file        4G     0B   -3

Next, make sure the swap is used after a reboot by adding a line to /etc/fstab that results in swapon -a to add the new swapfile to swap space:

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

If you wanted to remove the swap file from usage simply use swapoff:

sudo swapoff /swapfile

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