Explanation: Provisioned IOPS SSD (io1) volumes are designed to meet the needs of I/O-intensive workloads, particularly database workloads, that are sensitive to storage performance and consistency. An io1 volume can range in size from 4 GiB to 16 TiB and you can provision 100 up to 20,000 IOPS per volume. The maximum ratio of provisioned IOPS to requested volume size (in GiB) is 50:1. For example, a 100 GiB volume can be provisioned with up to 5,000 IOPS. Any volume 400 GiB in size or greater allows provisioning up to the 20,000 IOPS maximum.
Reference:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSVolumeTypes.html