How to enable SWAP on your server

Introduction

If you wish to enable or recreate SWAP on your server, there are a handful of methods that can be used. The below examples assume you are using CentOS or RedHat RHEL OS.

Procedure 

For Centos (swapfile):

First, determine if SWAP is enabled on the server already. The output from the following command can tell you.

SWAP not enabled:

# swapon -s
No Output

If SWAP is not enabled, then used these commands CREATE it.

Using example swap mount point: /root/cp-swap, first create the disk image.

dd if=/dev/zero of=/root/cp-swap bs=1M count=2000

Then, set permissions on the disk image.

chmod 0600 /root/cp-swap

Now, verify the file exists and has the correct permissions.

ls -al /root/cp-swap

Then, format the disk image.

mkswap /root/cp-swap

After that, enable the system to use the swap space.

swapon /root/cp-swap

Finally, verify swap is enabled by running this command.

swapon -s

To make the change permanent, add the following line to your /etc/fstab file.

/root/cp-swap swap swap defaults 0 0

Please use extreme caution when modifying the /etc/fstab file. It is responsible for the mounting of the partitions on the server. If unfamiliar with the file, we would recommend having a qualified System Adminstor make the change.

Where do I find a system administrator?

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If SWAP is already enabled, and you want to recreate it as a different size, you can use these commands:

First, check if SWAP is enabled:

SWAP is enabled:

# swapon -s
Filename Type Size Used Priority
/root/cp-swap file 1048572 107520 -2

Turn SWAP off so it can be recreated.

swapoff /root/cp-swap

Using example swap mount point: /root/cp-swap, recreate the disk image as a new size. Example 1G.

dd if=/dev/zero of=/root/cp-swap bs=1M count=1000

Then, set permissions on the disk image.

chmod 0600 /root/cp-swap

Now, verify the file exists and has the correct permissions.

ls -al /root/cp-swap

Then, format the disk image.

mkswap /root/cp-swap

Force the kernel to re-read the partition table so that it is not necessary to perform a reboot.

partprobe

After that, enable the system to use the swap space.

swapon /root/cp-swap

Finally, verify swap is enabled by running this command.

swapon -s

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RedHat Enterprise Linux (LVM)

RedHat has multiple ways of enabling swap. Please refer to their documentation for instructions.

RedHat - Swap Space

Resources

How to check available swap space on your server.