How do you configure a swap partition using cloud-init? - swap

Question

We have an instance that uses cloudinit for initial instantiation, and this instance and cloudinit work great.
We want to add swap to this instance, and have correctly configured a suitable disk, however we cannot figure out how to get cloudinit to initialise the swap disks, like cloudinit does with all the other disks on the machine.
Our configuration of our disks, including swap, is as follows:
fs_setup:
- label: vidi
device: /dev/xvde
filesystem: ext4
- label: swap
device: /dev/xvdg
filesystem: swap
mounts:
- [ /dev/xvde, /var/lib/vidispine, ext4, defaults, 0, 0 ]
- [ /dev/xvdg, none, swap, sw, 0, 0 ]
This results in an /etc/fstab as follows:
LABEL=cloudimg-rootfs / ext4 defaults,discard 0 0
/dev/xvde /var/lib/vidispine ext4 defaults,comment=cloudconfig 0 0
/dev/xvdg none swap sw,comment=cloudconfig 0 0
The disk /dev/xvde is formatted correctly on startup. The disk /dev/xvdg is ignored.
What addditional steps are required for cloudinit to "mkswap" and "swapon" the /dev/xvdg disk?

Answer 1

In response to "What addditional steps are required for cloudinit to 'mkswap' and 'swapon' the /dev/xvdg disk?", the short answer is "nothing".
The longer answer is that you need to be running a version of cloud-init with the following bugfix applied:
https://github.com/canonical/cloud-init/pull/143
Which fixes the following error when running mkswap:
mkswap: invalid block count argument: ''
Most specifically, Ubuntu Bionic images 20200131 and newer work properly.
Older versions of cloudinit require the following added to the runcmd scripts on boot to work around the bug above:
- mkswap /dev/xvdg
- swapon -a

Answer 2

Solution: swap is a space on the disk that is used when the amount of physical ram of memory is full, therefore it can save your system from crashing due to out of memory exception. In order to apply swap partition using cloud-init package on ubuntu, you'll need to mount dedicated disk partition ( in boot time ) on /etc/fstab ( a configuration table designed to ease the burden of mounting and unmounting file systems to a machine) create ( using mkswap ) and start swap ( using swapon ).
First create an attach an additional disk to your machine, I'm attaching example using terraform:
resource "aws_instance" "example" {
ami ="<some-ami>"
instance_type = "t3.micro"
tags = {
Name = "example"
}
// root
root_block_device {
volume_size = 50
volume_type = "gp2"
delete_on_termination = true
}
// swap partition
ebs_block_device {
device_name = "/dev/xvdb"
volume_size = 20
volume_type = "gp2"
delete_on_termination = true
}
}
Second mount an additional disk and toggle the swap functionality on template file of cloud-init:
mounts:
- [ /dev/nvme1n1, none, swap, sw, 0, 0 ]
bootcmd:
- mkswap /dev/nvme1n1
- swapon /dev/nvme1n1
For verification: type on terminal:
swapon --show
#output:
NAME TYPE SIZE USED PRIO
/dev/nvme1n1 partition 20G 0B -2