I have done a few times before but don’t have to do it very often, just noting it here.

Create an empty file. The following will create a 1GB file.:

# dd if=/dev/zero of=/var/swapfile bs=1024 count=1024000
1024000+0 records in
1024000+0 records out
1048576000 bytes (1.0 GB) copied, 10.9243 s, 96.0 MB/s

Change the permission:

# chmod 0600 /var/swapfile

Create the swap area:

# mkswap /var/swapfile
Setting up swapspace version 1, size = 1023996 KiB
no label, UUID=957c2a0c-30f9-4dd7-89ed-d88b9f471294

Enable the swap area:

# swapon /var/swapfile

Enable the swap area at boot by adding it to the /etc/fstab:

# vi /etc/fstab
…
/var/swapfile swap swap defaults 0 0
…

Disable the swap area:

# swapoff /var/swapfile