First, let us check what swap devices we have enabled. Check the NAME and SIZE columns of the following command:

$ swapon --show

If you ever installed a Linux distro, you may remember creating a separate swap partition. This, however, is only one method of creating swap space. The other is by adding a swap file:

$ sudo swapoff -a
$ sudo dd if=/dev/zero of=/swapfile bs=1024 count=$((4 * 1024 * 1024))
$ sudo chmod 600 /swapfile
$ sudo mkswap /swapfile
$ sudo swapon /swapfile

$ swapon --show

Just to clarify what we did:

• disabled all swap devices
• created a 4Gb zero-initialized file
• set the permission to the file so only root can edit it
• created a swap area from the file using mkswap (works on devices too)
• activated the swap area

The new swap area is temporary and will not survive a reboot. To make it permanent, we need to register it in /etc/fstab by adding a line such as this:

/swapfile swap swap defaults 0 0

Now that we created a swap file, what are the advantages / disadvantages when compared to a swap partition?

• advantages:
  • easier to manage
  • similar performance
• disadvantage:
  • can be affected by disk fragmentation (not the case for a partition)

In one terminal run vmstat and look at the swpd and free columns.

$ vmstat -w 1

In another terminal, open a python shell and allocate a bit more memory than the available RAM. Identify the moment when the newly created swap space is being used.

Notice that once you close the python shell and the memory is freed, swpd still displays a non-zero value. Why? There simply isn't a reason to clear the data from the swap area. If you really want to clean up the used swap space, try the following:

$ vmstat
$ sudo swapoff -a && swapon -a
$ vmstat