You have two options: add a swap partition or add a swap file. It is recommended to add a swap partition, but this is not possible if you don't have free Disk space available.
Its better to reboot the server to SINGLE USER mode(if possible) before adding the extra SWAP Space
And you can do an optional "swapoff" command to turn off all the swap spaces.
Create the swap partition fdisk.
# fdisk /dev/hda (Assuming Hard drive is hda)
Type n ( for new partition)
Determine the size for the partition
then Toggle the partiotion label to Linux Swap.Its id is 82
Save the partition table,exit fdisk and do a "partprobe" to make it effective
Now you have the swap partition, use the command mkswap to setup the swap partition.
At a shell prompt as root, type the following:
# mkswap /dev/hdaX
To enable the swap partition immediately, type the following command:
# swapon /dev/hdaX
To enable it at boot time, edit /etc/fstab to include:
/dev/hdaX swap swap defaults 0 0
The next time the system boots, it will enable the new swap partition.
After adding the new swap partition and enabling it, make sure it is enabled by viewing the output of the command
# cat /proc/swaps
Determine the size of the new swap file and multiply it by 1024 to determine the block size. For example, the block size of a 64 MB swap file is 65536.
At a shell prompt as root, type the following command with count being equal to the desired block size:
# dd if=/dev/zero of=/swapfile bs=1024 count=65536
Make it a SWAP file
# mkswap /swapfile
To enable the swap file immediately
# swapon /swapfile
To enable it at boot time, edit /etc/fstab and insert the following
/swapfile swap swap defaults 0 0