You need to use the dd command to create swap file. The mkswap command is used to set up a Linux swap area on a device or in a file.
Login as root user, and use the following command to create a swap file.
The following dd command example creates a swap file with the name “tmpDSK” under /backup directory with a size of 2000MB (2GB)
# dd if=/dev/zero of=/backup/tmpDSK bs=1M count=2000
Setup correct file permission for security reasons, enter:
# chown root:root /backup/tmpDSK
# chmod 600 /backup/tmpDSK
Make this file as a swap file using mkswap command.
# mkswap /backup/tmpDSK
Setting up swapspace version 1, size = 2097147 kB
To make this swap file available as a swap area even after the reboot, add the following line to the /etc/fstab file.
/backup/tmpDSK swap swap defaults 0 0
Enable the newly created swapfile.
# swapon /backup/tmpDSK
# free -m
total used free shared buffers cached
Mem: 4050 3222 828 0 210 2589
-/+ buffers/cache: 422 3628
Swap: 1999 0 1999