The Amazon post about the service disruption that happened late last month provides an interesting description of the behavior of the Amazon EC2 and EBS metadata datastores:
The EC2 and EBS APIs are implemented on multi-Availability Zone replicated datastores. These datastores are used to store metadata for resources such as instances, volumes, and snapshots. To protect against datastore corruption, currently when the primary copy loses power, the system automatically flips to a read-only mode in the other Availability Zones until power is restored to the affected Availability Zone or until we determine it is safe to promote another copy to primary.
Original title and link: The Behavior of EC2/EBS Metadata Replicated Datastore ( ©myNoSQL)