Shrinking a Windows boot drive in AWS

I recently deployed a couple of Windows domain controllers from a CloudFormation template and only after I'd done reasonable work to them did I realise that the boot drives were 100GB rather than a more sensible 40GB.

Now, that additional 60GB isn't exactly going to break the bank (it works out at about £50 per year) however I'm committed to provisioning the correct size. 

I investigated a few different options for shrinking the Windows boot drive in AWS and this is the one that I found to be the most clear.

In this walkthrough I'll be working on hostname DC01 as the Windows machine in question

AWS Console

• Take a snapshot of DC01

Windows

• Start -> Administrative Tools -> Computer Management -> Storage -> Disk Management
• Right-click on Volume C: -> Shrink Volume so the 'Total size after shrink' is no more than 39000 MB

• Start -> Shut down. 

AWS Console

•  Determine which Availability Zone DC01 is in. In this case it's zone 1a in eu-west:

• Create a new Amazon Linux EC2 instance in the same zone.
• I chose an EBS-optimised m1.large to improve the disk speed
• Detach the 100GB root volume from DC01
• Attach the 100GB root volume to the new Amazon Linux EC2 instance as device /dev/sdf (the default as of writing)
• Create a new 40GB standard EBS volume in the same zone and attach it to the Amazon Linux EC2 instance as device /dev/sdg (be careful - it defaults to /dev/sdf)

Amazon Linux EC2 instance

• Check that the devices are mounted correctly with 'fdisk':

• Yes, /dev/sdf is our 100G source drive and /dev/sdg is the new 40G destination
• Use 'dd' to copy the contents of the source to the destination
• dd if=/dev/sdf of=/dev/sdg bs=1M
• The 'bs=1M' tells dd to use a block size of one megabyte - this is not only faster but also reduces the number of I/O operations. AWS charges per I/O operation :)
• BONUS: this will also do a full block format to maximise EBS performance

• The 'no space left on device' message is perfectly normal - 100GB of data will not fit into 40GB - the important part is the useful data is occupying less than 40GB

AWS Console

• Stop the Amazon Linux EC2 instance
• Detach both the 100G and 40G volumes
• Attach the 40G volume as device /dev/sda1 of DC01
• Start DC01

Windows

• Verify that the instance comes up and the root device is 40GB as expected:

AWS Console

• delete the 100GB snapshot
• delete the 100GB original volume from DC01
• terminate the Amazon Linux EC2 instance

Tada!