IOPS as measured by Amazon

AWS doc - http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-io-characteristics.html
EBS benchmarking for piops -  - http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/benchmark_piops.html

"IOPS are input/output operations per second. Amazon EBS measures each I/O operation per second (that is 256 KB or smaller) as one IOPS. I/O operations that are larger than 256 KB are counted in 256 KB capacity units. For example, a 1,024 KB I/O operation would count as 4 IOPS. When you provision a 4,000 IOPS volume and attach it to an EBS-optimized instance that can provide the necessary bandwidth, you can transfer up to 4,000 chunks of data per second (provided that the I/O does not exceed the 128 MB/s per volume throughput limit of General Purpose (SSD) and Provisioned IOPS (SSD) volumes).

This configuration could transfer 4,000 32 KB chunks, 2,000 64 KB chunks, or 1,000 128 KB chunks of data per second as well, before hitting the 128 MB/s per volume throughput limit. If your I/O chunks are very large, you may experience a smaller number of IOPS than you provisioned because you are hitting the volume throughput limit. For more information, see Amazon EBS Volume Types.

For 32 KB or smaller I/O operations, you should see the amount of IOPS that you have provisioned, provided that you are driving enough I/O to keep the drives busy. For smaller I/O operations, you may even see an IOPS value that is higher than what you have provisioned (when measured on the client side), and this is because the client may be coalescing multiple smaller I/O operations into a smaller number of large chunks.

If you are not experiencing the expected IOPS or throughput you have provisioned, ensure that your EC2 bandwidth is not the limiting factor; your instance should be EBS-optimized (or include 10 Gigabit network connectivity) and your instance type EBS dedicated bandwidth should exceed the I/O throughput you intend to drive."