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Setup Swap File on Linux

Published on: Thu, Oct 2, 2014 at 3:25 pm EST

There will be times where you need to increase the responsiveness of your server to prevent out of memory issues. Out of memory issues happen when an application running on your server starts consuming a large amount of memory. Swap is designed as virtual memory, which uses your harddrive to store data that cannot be held in RAM. This tutorial will show you how to create a swap file, which should work under Ubuntu, CentOS, and Debian. This tutorial is not meant for any Custom ISO, but it is possible to follow along.

Step 1: Verify that swap does not exist

To prevent any issues during this tutorial, you will need to run the following to verify that a swap space is currently not active:

free -m

After running that command you should see something similar to this output:

total              used       free     shared    buffers     cached
Mem:               1840       1614     226       15          36       1340
-/+ buffers/cache:            238      1602
Swap:              0          0        0

If you see a value of 0 in the Swap section, then you can proceed to step 2.

Alternatively, you can run the following command to see if there is a configured swap file:

swapon -s

If you do not see any output from swapon, then proceed to step 2.

Step 2: Create swap file

You will need to choose a location for your file. In this tutorial, it will be stored at the root of the server. We will create a 2GB swap file by running the following command:

dd if=/dev/zero of=/swapfile count=2048 bs=1M

The dd command will produce output in a similar format to:

2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 10.5356 s, 204 MB/s

Next, verify that the file is located at the root of your Vultr VPS by running:

ls / | grep swapfile

Proceed if you see the swapfile file.

Step 3: Activate the swap file

Swap files are not recognized automatically. We will need to tell the server how to format the file and enable it so it can be used as a valid swap file. As a security measure, update the swapfile permissions to only allow R/W for root and no other users. Run:

chmod 600 /swapfile

The permission change can be verified by running the following command:

ls -lh /swapfile

You will see a file display:

-rw------- 1 root root 2.0G Oct  2 18:47 /swapfile

Next, tell the server to setup the swap file by running:

mkswap /swapfile

After running it, you will see the following output:

Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=ff3fc469-9c4b-4913-b653-ec53d6460d0e

If everything is shown as above, you are now ready to move on to the next step.

Step 4: Turn swap on

Once your file is ready to be used as swap, you need to enable it by running:

swapon /swapfile

You can verify that the swap file is active by running the free command again.

free -m

total       used       free     shared    buffers     cached
Mem:          1840       1754         86         16         23       1519
-/+ buffers/cache:        210       1630
Swap:         2047          0       2047

If Swap shows something other than 0, then you have successfully setup swap.

Step 5: Enable swap on reboot

By default, your server will not automatically enable this new swap file. To enable it on boot, you can update the /etc/fstab file. Any text editor will suffice. In this example, I will be using nano.

nano /etc/fstab

Add the following line at the end of the file:

/swapfile   none    swap    sw    0   0

Save and close when you are finished editing the file. We are all done!

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