Specify Multiple Availability Zones in a Single EC2 Spot Fleet Request
You can now specify multiple Availability Zones and VPC subnets in a single Amazon EC2 Spot fleet launch specification. Because the Spot price fluctuates independently for each Availability Zone, listing more availability zones can enable you to run your instances at even lower prices. By default, Spot instances will be placed into the zones where the Spot price is lowest at the time of launch.
Optionally, you can choose the “diversified” allocation strategy to distribute your instances evenly across all the zones and subnets you specified, enabling you to improve fault tolerance and application availability. You can include up to a maximum of 50 AZ/subnet combinations and 50 launch specifications per request.
To get started, visit Spot Fleet Requests in the Amazon EC2 User Guide at: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-fleet-requests.html