Decrease the volume size of an EBS

Using pure native tools you can only increase the size of your EBS volume, not decrease it. To decrease it we need to make a new volume then copy the data over. Please take additional considerations in case the volume is bootable, if so test for booting functionality in a test copy before committing to prod.

Typically I would:

• -~~Take a snapshot of the volume after shutting down the instance. ~~(As Leo noted in the note, this is not needed in this case since it wouldn't help.)
• -Use the snapshot to create a new volume to match the size (If it doesn't fit under 100GB you will have to manage data yourself to get to your ideal size of 100GB)
• -Attach it to the instance
• -Depending on our OS, use your appropriate tool to ensure it is recognized by the OS such as creating partitions using fdisk on Windows.
• -Copy all teh data.
• -Again if we are booting from this volume ensure you handle your boot loaders in this case.
• -Finally shutdown the instance to deattach the old drives, handle any bootable drive items such as making it bootable.
• -Reboot

Ensure you test this if you are on production on a non production environment if possible.

For more information check out these docs: https://docs.aws.amazon.com/ebs/latest/userguide/requesting-ebs-volume-modifications.html