Preserve Existing Partition Sizes with Cloud-Init Auto Install on Ubuntu 24.04.1

Background

When you use Cloud-Init for auto-installing Ubuntu, it creates new partitions for the root filesystem and swap space by default. This behavior can be problematic if you want to preserve the existing partition sizes. For instance, if you have already set up your system with specific partition sizes, you might not want to resize them during the installation process.

Solution

To preserve the existing partition sizes during the Cloud-Init installation process, you can use the cloud-config file to define the partition sizes. Here's how to do it:

Create a Cloud-Config File

First, you need to create a cloud-config file. You can create this file on your local machine or on the cloud instance using SSH. The file should be located in the root directory of the instance, typically at /cloud-config.yaml or /cloud-config.json. In this example, we will use a YAML file.

Define Partition Sizes

Next, you need to define the partition sizes in the cloud-config file. Here's an example of how to define the root filesystem and swap partition sizes:

 disk_size: 20GB
 root_partition:
 size: ${disk_size}
 overwrite_files: [/etc/fstab, /etc/crypttab]
 partitions:
 - size: 10GB
 name: swap
 boot: true
 format: swap
 
 

In this example, we define a disk size of 20GB and set the root partition to use the entire disk, except for the swap partition. We also define a swap partition of size 10GB.

Install Cloud-Init

Before you can use the cloud-config file, you need to install Cloud-Init on your Ubuntu instance. You can install it using the following command:

 sudo apt-get update
 

Then, install Cloud-Init:

 sudo apt-get install cloud-init
 

Configure Cloud-Init

Once you have installed Cloud-Init, you need to configure it to use the cloud-config file. You can do this by creating a file named user-data in the root directory of the instance. This file should contain the following:

 #!/bin/bash
 set -o nounset
 set -o errexit
 set -o pipefail
if [ -f /cloud-config.yaml ]; then
cloud-config --no-overwrite config=$(/path/to/your/cloud-config/file)
fi

Replace /path/to/your/cloud-config/file with the path to your cloud-config file. This script will be executed during the boot process and will apply the configuration defined in the cloud-config file.

Reboot Instance

Finally, reboot the instance to apply the changes:

 sudo reboot
 

Summary

• Cloud-Init resizes partitions by default during installation, but you can preserve existing partition sizes by defining them in a cloud-config file.
• Create a cloud-config file with the desired partition sizes and install Cloud-Init on the Ubuntu instance.
• Configure Cloud-Init to use the cloud-config file during the boot process and reboot the instance to apply the changes.