Increasing Boot Disk Size via Terraform: A Complete Guide

Technical writer

Terraform

20.08.2025

Reading time: 5 min

Terraform is one of the most effective tools for working with the IaC (Infrastructure as Code) model. This open-source software makes it much easier to deploy and manage infrastructure both in local environments and in the cloud.

In this article, we will look at how to increase the size of a virtual machine’s boot disk using Terraform, in different ways and across different environments.

How to Create Boot Disks in Terraform

Terraform offers a number of tools for working with virtual machine disks. However, before you can increase a disk size, you first need to create it.

When creating a virtual machine, you can immediately specify the disk size using the size parameter. For example, to create a disk with a size of 100 GB, you need to add the line size = 1024 * 100 to your Terraform configuration.

The configuration file has a .tf extension and is located in the root directory. For example, if you want to create a virtual machine in Hostman and set its hard disk size to 100 GB, then in the .tf configuration file you need to create a hm_server resource (for an additional disk, you would create hm_server_disk) and specify this parameter in the configuration block.

Example for an additional disk:

resource "hm_server" "my-server" {
  name = "My Server Disk"
  os_id = data.hm_os.os.id

  configuration {
    configurator_id = data.hm_configurator.configurator.id
    size  = 1024 * 100
  }
}

In this example, we set the disk size to 100 GB. The size is always specified in megabytes, with increments of 5120 MB (5 GB). Naturally, you can adjust parameter values according to your project’s needs.

To attach the disk to a virtual machine, use the source_server_id parameter in the disk block:

resource "hm_server_disk" "my-server-disk" {
  disk {
    size = 1024 * 10
    source_server_id = hm_server.my-server.id
  }
}

Note that the id value will be automatically assigned to the resource after it is successfully created.

How to Increase the Boot Disk Size of a Virtual Machine in Terraform

Changing the size of a Hostman disk in Terraform is straightforward.

Here’s an example for our newly created system disk:

configuration {
    configurator_id = data.hm_configurator.configurator.id
    size  = 1024 * 200
}

Now the disk size will be 200 GB, and after rebooting, the filesystem will also expand.

The main thing is not to forget to update your Terraform configuration using:

terraform apply

So the changes are applied to your infrastructure.

You can only increase disk sizes (not decrease), but you can also delete existing disks and add new ones.

Increasing Disk Size with Terraform in GCP

Google Cloud Platform (GCP) is a cloud platform offering many tools and services for developing, deploying, and managing applications in the cloud.

The instructions given above will work for GCP as well; you just need to replace hm_server with google_compute in the first line of the resource.

Example:

resource "google_compute" "mydisk" {
  name = "my_new_VM"
  type  = "ssd"
  size  = 100
}

However, GCP also allows you to create a disk via Terraform whose size can be changed later without having to create a new disk and copy data from the old one.

You can do this by adding the following lines:

resource "google_compute" "mydisk" {
  name  = "my_new_VM"
  image = data.google_compute_image.my_image.self_link
}

Here we added a line with the image data. This enables dynamic changes to the disk parameters without using initialization parameters that are intended for recreating the disk rather than modifying it.

Increasing Disk Size with Terraform in EBS

EBS (Amazon Elastic Block Store) is a data storage service in Amazon Web Services (AWS). It provides block storage that can be used for virtual machines and data storage.

EBS volumes can be resized, and Terraform greatly simplifies this task. With Terraform, you can change an EBS disk size in just three steps.

Step 1 — Create a Terraform Resource

The code will be similar to the previous examples, but with EBS-specific values and some additional parameters:

resource "aws_ebs_volume" "mydisk" {
  zone = "europe-north1-a"
  size = 200
  type = "ssd"
  tags {
    Name = "mydisk"
    Role = "db"
    Terraform = "true"
    FS = "xfs"
  }
}

Then import the volume (name given as an example):

terraform import aws_ebs_volume.mydisk vol-13579ace02468bdf1

If the import is successful, you’ll get a confirmation message.

Now you can change the volume size by replacing 200 with 500 in the code:

size = 500

Then run:

terraform apply -target=aws_ebs_volume.mydisk

After that, you should see a message confirming the volume size change.

Step 2 — Get the IP Address

Create an instance and retrieve its identifier:

data "aws_instance" "mysql" {
  filter {
    name   = "block-device-mapping.volume-id"
    values = ["${aws_ebs_volume.mydisk.id}"]
  }
}

output "instance_id" {
  value = "${data.aws_instance.mydisk.id}"
}

Update the configuration with:

terraform apply
terraform refresh

This should produce output like:

instance_id = i-13579ace02468bdf1

Next, get the mount point pointing to our volume inside the instance:

locals {
  mount_point = "${data.aws_instance.mydisk.ebs_block_device.0.device_name}"
}

Step 3 — Run the Script

To make the OS recognize and use the entire expanded disk size, create a script like this:

resource "null_resource" "expand_disk" {
  connection {
    type        = "ssh"
    user        = "username"
    private_key = "${file("ssh-key-here")}"
    host        = "${data.aws_instance.mydisk.xxx.xxx.xxx.xxx}"
 }

  provisioner "remote-exec" {
    inline = [
      "sudo lsblk",
      "sudo xfs_growfs ${local.mount_point}",
    ]
  }
}

Note: replace xxx.xxx.xxx.xxx with the public IP address of the created disk.

Finally, run:

terraform apply -target=null_resource.expand_disk

This way, you can resize EBS volumes in Terraform (not only increase, but also decrease) without creating a new volume, which is not always convenient.

Conclusion

We’ve learned how to create disks in Terraform and increase their size using Terraform itself, as well as configuration files and scripts in different environments.

Terraform

20.08.2025

Reading time: 5 min

Similar

Terraform

Increasing Boot Disk Size via Terraform: A Complete Guide

How to Create Boot Disks in Terraform

How to Increase the Boot Disk Size of a Virtual Machine in Terraform

Increasing Disk Size with Terraform in GCP

Increasing Disk Size with Terraform in EBS

Step 1 — Create a Terraform Resource

Step 2 — Get the IP Address

Step 3 — Run the Script

Conclusion

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