Decrease FSx Size

To clarify the accepted answer, specifically this sentence " To decrease, you'll need to make a new fsx and reload from backup." The words reload from backup can cause a bit of confusion because the FSx for windows console does not allow one to change storage capacity when doing a restore from backup.

We are not actually restoring from an FSx backup at all. Simply copy the data from 8TB to the newly created 3TB filesystem.

In summary:

1. Take a backup of 8TB for safety
2. Create a new Amazon FSx for Windows File System, 3TB
3. Use AWS DataSync Service or Robocopy to move the data from the 8TB to the 3TB
4. Recreate the SMB shares and permissions
5. Create any alias or SPNs
6. Terminate the 8TB and use the 3TB
7. The full workflow can be found in link [1]

You can automate the migration to a smaller file system using link [2]. The title says upgrade SingleAZ to MultiAZ but the same concept applies, and it can also be used to move between two SingleAZ systems. It moves data, CNAME records, SPN\alias and share permissions ACLs, etc.. from SingleAZ to another SingleAZ FSx as well.

In the CloudFormation example, I created a DataSync agent (EC2 instance) to cater for all migration scenarios including cross region, cross account migrations but you can change that part of the code to be a normal DataSync source and destination location if needed.

References:

[1] https://docs.aws.amazon.com/fsx/latest/WindowsGuide/migrate-files-fsx.html

[2] https://aws.amazon.com/blogs/modernizing-with-aws/automate-the-upgrade-of-an-amazon-fsx-for-windows-file-server-to-a-multi-az-deployment/