Swap in Linux

[kuldeep]

Swap space is used when the amount of physical memory (RAM) is full. If the system needs more memory and no more RAM is available, inactive pages in memory are moved to the swap space. Swap should not be considered as a replacement of RAM memory because swap space is on hard drives and I/O access to hard drives is slower than I/O access memory.

The swap must be located on a dedicated swap partition and it is designed to help RAM memory not to replace it.
Swap partition

* Using parted a swap partition can be created in order to be added to the existing swap (or just to create it for the first time) :

$ parted /dev/sdb
GNU Parted 2.1
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mkpart
Partition type? primary/extended? p
File system type? [ext2]? linux-swap
Start? 1
End? 512M
(parted) quit
Now /dev/sdb1 is a 512M swap partition

* Next step is create the swap filesystem on the swap partition :

$ mkswap /dev/sdb1
Setting up swapspace version 1, size = 498684 KiB
no label, UUID=81631aa8-8064-47fc-92ef-5eb3fa6e8b87

* Once the swap filesystem has been created the final step is activate the new swap space :

$ free | grep Swap
Swap: 1736696 0 1736696
The initial swap size is 1736696 Kb

$ swapon /dev/sdb1
It activates the new swap

$ free | grep Swap
Swap: 2235376 0 2235376

After the new swap (512 M) has been added the new swap size is 2.2G

* Adding the corresponding line to /etc/fstab the system will activate automatically the swap on boot :

$ echo "/dev/sdb1 swap swap defaults 0 0" >> /etc/fstab