The following dd command example creates a swap file with the name “swapfile” under /root directory with a size of 8192MB (8GB).

To create the swap space (non-persistent):

# dd if=/dev/zero of=/root/myswapfile bs=1m count=8192;
8192+0 records in
8192+0 records out

Change the permission of the swap file so that only root can access it.

# chmod 0600 /root/myswapfile;

Make this file as a swap file using mkswap command.

# mkswap /root/myswapfile

Enable the newly created swapfile.

# swapon /root/myswapfile

To make this swap file available as a swap area even after the reboot, add the following line to the /etc/fstab file.

# cat /etc/fstab
/root/myswapfile               swap                    swap    defaults        0 0

Verify whether the newly created swap area is available for your use.

# swapon -s
Filename                        Type            Size    Used    Priority
/dev/sda2                       partition       4192956 0       -1
/root/myswapfile                file            1048568 0       -2

# free -k
             total       used       free     shared    buffers     cached
Mem:       3082356    3022364      59992          0      52056    2646472
-/+ buffers/cache:     323836    2758520
Swap:      5241524          0    5241524

Note: In the output of swapon -s command, the Type column will say “file” if the swap space is created from a swap file.

If you don’t want to reboot to verify whether the system takes all the swap space mentioned in the /etc/fstab, you can do the following, which will disable and enable all the swap partition mentioned in the /etc/fstab

# swapoff -a

# swapon -a