How can I create a swap file?

Question

I know how to create and use a swap partition but can I also use a file instead?

How can I create a swap file on a Linux system?

Answer 1

Let it be clear, that there is a little overhead when dealing with a file instead of a partition.

Further, I myself have on several machines a swap file on mdadm RAID, therefore there's yet another overhead. But if you adjust vm.swappiness wisely to a more acceptable value than 60, which is the default, you should have no problem.

For instance, I have 32GB RAM server with 32GB swap file on RAID6 with vm.swappiness = 1. Quoting the Wikipedia:

vm.swappiness = 1: Kernel version 3.5 and over, as well as Red Hat kernel version 2.6.32-303 and over: Minimum amount of swapping without disabling it entirely.

In this example, we create a swap file:

• 8GB in size

• Located in /raid1/

Change these two things accordingly to your needs.

1. Open terminal and become root (su); if you have sudo enabled, you may also do for example sudo -i; see man sudo for all options):

   sudo -i
   

2. Allocate space for the swap file:

   dd if=/dev/zero of=/raid1/swapfile bs=1G count=8
   

   Optionally, if your system supports it, you may add status=progress to that command line.

   Note, that the size specified here in G is in GiB (multiples of 1024).

3. Change permissions of the swap file, so that only root can access it:

   chmod 600 /raid1/swapfile
   

4. Make this file a swap file:

   mkswap /raid1/swapfile
   

5. Enable the swap file:

   swapon /raid1/swapfile
   

6. Verify, whether the swap file is in use:

   cat /proc/swaps
   

7. Open a text editor you are skilled in with this file, e.g. nano if unsure:

   nano /etc/fstab
   

8. To make this swap file available after reboot, add the following line:

   /raid1/swapfile        none        swap        sw        0        0