Background
Here are bare minimal instructions on how to set up a swap file for Ubuntu Linux and CentOS. The basic difference between Ubuntu and CentOS is that with Ubuntu fallocate seems to always work and on CentOS dd seems to work better.
In the beginning and in the end swapon command is used to test for the availability of the swap file. In the middle the vi text editor is used to append the requisite swap entries to fstab. If you’re working with AWS EC2, also see the reference pointing to their site.
It’s possible to use this entire procedure for CentOS but you can’t do fallocate and you have to use dd instead.
Ubuntu Instructions
Below if you have 2GB RAM and you want to create the maximum size swapfile as per the table at the bottom of this article. On 4GB you could add a maximum of 8GB.
Go in as sudo -i
swapon --show fallocate -l 4G /swapfile chmod 600 /swapfile mkswap /swapfile swapon /swapfile
vi /etc/fstab
Append or comment existing swap entry and then append this:
/swapfile swap swap defaults 0 0
Check your work:
swapon --show free -h
CentOS 7
fallocate doesn’t work well with CentOS so use dd as per below.
Example is for 4 GB, but for 1 GB count = 1048576 so use a calculator and multiply by your desired size.
To assess the situation I like to use free -m or swapon --show
dd if=/dev/zero of=/swapfile bs=1024 count=4194304 chmod 600 /swapfile mkswap /swapfile swapon /swapfile vi /etc/fstab
Add this to fstab:
/swapfile swap swap defaults 0 0
Comment out the old swap file line.
Note about CentOS and dedicated swap partitions
If you use vi to edit the /etc/fstab file, and you encounter the line below:
/dev/mapper/cl-swap swap swap defaults 0 0
That basically means the installation engineer created a seperate swap partition for your installation. Although this is technically a more performant way of using swap, it completely breaks the ability to easily upgrade swap space. The reason for this is you would have to go and find the backend technology for the disk (e.g. the VM or physical host type) and then fiddle with complicated commands to enlare it. Also, if you just comment out that line and use the hard disk type swap option as per our advice, you will end up with an orphaned partition of wasted disk space.
The solution is too complicated to document here, as there are too many different types of disk backends, but you could use this document as a primer.
Note about existing swap file
There is a command swapoff
but the problem is you shouldn’t just use it unless you have a lot of time. See references why this is complicated, but in a nutshell once you run this command it’s going to transfer what’s in the current swap file to memory, and that’s a complicated process. It might just crash your machine or slow it down to FUBAR.
So it’s better just to create a new swap file, and then remove the other swap file later. Follow the above steps but use /swapfile2 as the name.
How Much Swap Is Needed*
*Adapted from the Ubuntu reference, removed hibernation column as Servers don’t hibernate
RAM Recommended Maximum 1GB 1GB 2GB 2GB 1GB 4GB 3GB 2GB 6GB 4GB 2GB 8GB 5GB 2GB 10GB 6GB 2GB 12GB 8GB 3GB 16GB 12GB 3GB 24GB 16GB 4GB 32GB 24GB 5GB 48GB 32GB 6GB 64GB 64GB 8GB 128GB 128GB 11GB 256GB 256GB 16GB 512GB 512GB 23GB 1TB 1TB 32GB 2TB 2TB 46GB 4TB 4TB 64GB 8TB 8TB 91GB 16TB
References
- https://www.howtogeek.com/455981/how-to-create-a-swap-file-on-linux/
- https://help.ubuntu.com/community/SwapFaq#How_much_swap_do_I_need.3F
- https://linuxize.com/post/how-to-add-swap-space-on-centos-7/
- https://linuxize.com/post/how-to-add-swap-space-on-ubuntu-18-04/
- https://aws.amazon.com/premiumsupport/knowledge-center/ec2-memory-swap-file/
- https://unix.stackexchange.com/questions/403451/why-is-swapoff-so-slow