EBS Snapshots occur _____

A, Asynchronously

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-creating-snapshot.html

Snapshots occur asynchronously; the point-in-time snapshot is created immediately, but the status of the snapshot is pending until the snapshot is complete (when all of the modified blocks have been transferred to Amazon S3), which can take several hours for large initial snapshots or subsequent snapshots where many blocks have changed. While it is completing, an in-progress snapshot is not affected by ongoing reads and writes to the volume.