How to Add Swap Space to a Running Linux System

Why?

While swap is critically important when you’re low on memory, it’s not immediately obvious that Linux systems really want at least a modest amount of swap space even when you have lots and lots of ram. This allows the kernel to push out unused data, freeing up memory to do a better job caching disk access.

How Much?

Generally 1GB or more. If you’re extremely low on disk space, try to come up with at least a 512MB or 256MB swap.

Going up to 4GB is a good idea on a larger server. Going a good deal beyond that probably won’t add much benefit (except in special cases where you know you’ll have lots of idle processes).

Checking Existing Memory Settings

To find out how much memory you have installed, run this:

sudo lshw -short -C memory

The line with “System memory” is the one you care about. Here are some example output lines from systems with 8GB and 4GB of memory, respectively:

/0/5 memory 7811MiB System memory
/0/1000 memory 4GiB System Memory

BTW, lshw is great at reporting on all your main system specs; try this at some point:

sudo lshw -short | less

To see how much swap space you have, run:

cat /proc/swaps

Sample output:

cat /proc/swaps

Filename Type Size Used Priority

/var/swap/swap1 file 1048572 6952 -1

The Size column is in kilobytes, so that’s a 1GB swap space.

Alternatively, run “top” and look at the 5th line of output; the number immediately following “Swap:” is the swap space size. If the line starts with “KiB Swap:”, this number is in kilobytes, or if it starts with “MiB Swap:”, the number is in megabytes.

If there are no lines other than the banner line, you have no swap space and should do the following.

Adding Swap

If you’ve broken up your disk into small partitions and “/var/” is low on space, you’re welcome to put the swap directory somewhere else, adjusting the following commands to match.

sudo mkdir -p /var/swap

Make sure there isn’t already a “swap1” file; if there is, use a different name:

ls -al

The “dd” command will make a 2048 * 1MB = 2GB swap space. Adjust 2048 up or down as needed.

sudo dd if=/dev/zero of=/var/swap/swap1 bs=1048576 count=2048

Now format that file as swap space and make it “live”, available for immediate use:

sudo chown root.root /var/swap/swap1
sudo chmod 600 /var/swap/swap1
sudo mkswap /var/swap/swap1
sudo swapon /var/swap/swap1

Check that it’s activated. You should see a new line in the following output:

cat /proc/swaps

You should also see this swap space available when running top; look at the value following “Swap:” on the 5th line.

Once this is done, bring up /etc/fstab in your favorite editor under sudo as well to make the change permanent. Please type carefully – recovering from a typo in fstab can be very complicated.

sudo nano /etc/fstab

The line to add is:

​​/var/swap/swap1 swap swap defaults 0 0

The spacing between elements isn’t critical, but please double check that you have all components, there are no spaces in the file name, and there’s a linefeed at the end of the file. This will bring up your swap automatically on the next boot. Save and exit. (You do not need to reboot to activate the swap; the “swapon” command above did that, and the fstab line is only used for the next boot.)

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