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How to increase swap file size

Posted on 2 mins read

We had a couple of servers running whose swap files needed to be increased. These are the steps I took to add more swap space.

Let’s start by checking where our current swap file is located and how big it is:

swapon -s

Its output will look something like this:

Filename      Type  Size	Used	Priority  
/mnt/swap.1   file  524288	0	    -1

Our swap file is located at /mnt/swap.1 and is currently 512M big. Before we continue, let’s disable all swap files:

swapoff -a 

Now we overwrite the existing swap file to increase its size with the dd command. In this example, I’m creating a file that is 1GB in size (1048576 blocks of 1024 bytes):

dd if=/dev/zero of=/mnt/swap.1 bs=1024 count=1048576

An alternative to the dd command is fallocate, which is a lot faster since it only preallocates the space on the file system instead of writing zeroes into it. The command looks like this:

fallocate -l 1G /mnt/swap.1

Note: fallocate is not suitable for every file system, as pointed out in the swapon Ubuntu man pages so make sure it will work properly on your system:

You should not use swapon on a file with holes. This can be seen in the system log as

swapon: swapfile has holes.

The swap file implementation in the kernel expects to be able to write to the file directly, without the assistance of the filesystem. This is a problem on preallocated files (e.g. fallocate(1)) on filesystems like XFS or ext4, and on copy-on-write filesystems like btrfs.

Let’s continue by setting up the file as a swap file:

mkswap /mnt/swap.1

And now tell the system to start swapping to it:

swapon /mnt/swap.1

That’s it! We can verify that the new swap file is being used using the free command:

free -m

You should see the “Swap:” line at the end of its output:

             total       used       free     shared    buffers     cached
Mem:           988        854        134         62         12        210
-/+ buffers/cache:        631        356
Swap:         1023          0       1023