April 25th, 2017
CloudFormation: Automating new LVM volumes for EC2 instances
By Doug I

Introduction

Creating a separate EBS volume can be very useful to prevent application data loss in the event your EC2 instance is unexpectedly terminated. It also makes it very easy to back up your application data by snapshotting the EBS volume.

AWS makes it easy to add additional EBS volumes to an EC2 Instance in CloudFormation templates; however, it’s not obvious from the AWS CloudFormation docs how to map an EBS volume to a Linux mount point, like /var/myapp.

In this article, I’ll show you how to add an EBS volume to an EC2 instance and automatically mount the EBS volume to a directory/mount point inside the instance.

Deploying the CloudFormation template

The example CloudFormation template we’ll be using is at 1Strategy/blog-add-ebs-volume.

If you’d like to try it out, clone the git repo and run this AWS CLI command to deploy it:

aws cloudformation deploy --template-file ./lvm-volume.yaml --stack-name lvm-volume --parameter-overrides VpcIdParameter=vpc-abcd1234 InstanceSubnetIdParameter=subnet-abcd1234 SshKeyParameter=mysshkey

How to add an EBS volume in CloudFormation

EBS volumes are block storage devices, and adding a block device to an instance requires only a few lines of code in CloudFormation.

AWS Block Device naming conventions

In the example above, I’ve named the block device /dev/sdf. See Device Naming on Linux instances for more details on naming conventions for Linux block devices. In short, you should use sdf - sdp.

Note that the OS, e.g. Amazon Linux, will rename /dev/sdf to /dev/xvdf (Xen Virtual Device “F”). For an explanation, see Device Name Considerations in the AWS EC2 User Guide.

EBS Volume Attached But Not Usable

If you deploy your CloudFormation template with this block device mapping, the EBS volume will be attached to your EC2 instance, but you won’t be able to use it until you create a file system on it and mount it to a directory, or mount point, e.g. /var/myapp.

 

How to mount a volume in Linux automatically

You can, of course, SSH in to your instance after creation and run the commands to create and mount a file system by hand, but if you are already using CloudFormation you want it all automated. In my case, I wanted to mount the EBS volume under /var/myapp so my application could store its data on a separate volume which wouldn’t disappear if the instance was terminated.

Running File System Create and Mount commands

The CloudFormation snippet below uses AWS CloudFormation::Init, combined with cfn-init in the UserDatasection to run the commands to create and mount the file system at boot.

Note that the commands are run in alphabetical order by name, not in the order listed in the commands section. That is why I’ve prepended each command with a number, e.g. 1_pvcreate, 2_vgcreate, etc.

LVM

I’m using LVM, or Logical Volume Manager, to make it easier to resize logical disk volumes in the future if need be.

The AWS::CloudFormation::Init commands below first create an LVM physical volume, then a volume group, then a logical volume.

File System

After that, we create an ext4 file system, make the /var/myapp directory, and add the mount point to the /etc/fstabfile so it will be mounted every time the system boots.

Finally, we run the mount -a command to mount the newly added logical volume to the /var/myapp directory.


UserData -> cfn-init -> AWS::CloudFormation::Init

In the snippet above, the UserData section is run once at OS first boot as a Bash script. It calls cfn-init which triggers the AWS::CloudFormation::Init section in the Metadata section above. This is the recommended pattern for running bootstrap commands on your Linux instance. See AWS::CloudFormation::Init for more details.

Note

If you are installing an application package which will create directories under your /var/myapp mount point, be sure to run the commands to create the mount point before installing your package. Otherwise the sub-directories the package creates will be wiped out.

Results

After running the complete CloudFormation template you will have the /var/myapp directory mounted on your /dev/sdf (or /dev/xvdf) EBS volume. If your instance disappears, your data will not be lost. You can spin up another EC2 instance and mount the orphaned volume to recover your data. Note that once you’ve created a file system and LVM volumes you won’t need to create them again for that EBS volume.

You can view information about your EBS volume and mount point by running the lsblk command on your instance. Run man lsblk for more info.


Summary

The full example CloudFormation template is at 1Strategy/blog-add-ebs-volume.

Creating a separate EBS volume can be very useful to prevent application data loss in the event your EC2 instance is unexpectedly terminated. It also makes it very easy to back up your application data by snapshotting the EBS volume. Finally, it cleanly separates your application data from your OS root volume.

In this post, I showed you, via a AWS CloudFormation template, how to create an EC2 Instance with an EBS volume to store your application data separately from your EC2 instance’s root volume.

You also learned how to create an LVM volume and ext4 filesystem on your EBS volume, and how to auto-mount the file system at boot by adding a line to your /etc/fstab file.

I hope this helps you improve your CloudFormation templates when dealing with EBS volumes for applications.