How to Create a Swap File

A swap file is an ordinary file that is in noway special to the kernel. The purpose of the swap file is to reserve the disk space so that the kernel can quickly swap out a page without having to go through all the things that are necessary when allocating a disk sector to a file.Because a hole in a file means that there are no disk sectors allocated (for that place in the file), it is not good for the kernel to try to use them.

In order to create the swap file without holes, use the following command:

1.) Assuming we want to create a swap file with 2GB size (1024 x 1024 x 2 = 2097152).

[bash]# dd if=/dev/zero of=/swapfile bs=1024 count=2097152

     where if is source, of is output file for dd to write to which is /swapfile in this case, bs is read/write byte size at a time and count is number of blocks.

2.) The next step is to make it a swap file

[bash]# mkswap /swapfile

3.) Activate the swap file

[bash]# swapon /swapfile

4.) Check the newly created swap space usinf free or top

[bash]# free -m

     or 

[bash]# top

5.) Write it to fstab to make it available for the system even after a reboot

[bash]# echo swapfile swap swap defaults 0 0 >> /etc/fstab

 

**) Using cat give you the following output:

[bash]# cat /etc/fstab
/swapfile1 swap swap defaults 0 0

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