AWS uses names such as us-west-1a to refer to a given availability zone (AZ) in a region. What you may not know is that AZ naming is intentionally inconsistent across accounts. In other words, the us-west-1a in the Konekti AWS production account is not necessarily the same as the us-west-1a in your account. AWS uses this to load balance workloads across accounts. Otherwise, the “1a” AZ in a region would receive more than its share of the load. This nondeterministic naming also allows AWS to add AZs in a way that new accounts will automatically begin using them.

Why is this important? An AWS AZ consists of a building or set of buildings. Latency is a critical factor in ensuring high throughput for intra-VPC traffic.

Measuring with ping is insufficient for this level of granularity. Use a better tool such as qperf.

AWS can’t increase the speed of light in fiber; however, it can optimize the network stack on its instances. The new C5 instance type has an average intra-AZ latency of 50 microseconds. I confirmed this value using qperf and the c5.9xlarge instance type. The latency between my us-east-1a and us-east-1b clocked in around the 500 microsecond mark.

For more on what AWS is doing to increase network performance in the VPC, check out The Floodgates Are Open – Increased Network Bandwidth for EC2 Instances.

If you assumed that cross-account EC2 instances located in “us-east-1a” were in the same physical AZ, you could unknowingly incur the inter-AZ latency penalty. In addition, you might make incorrect assumptions about redundancy if you are building cross-account applications.

Apparently, there is an exception to the AZ naming. The us-east-1f appears to be consistent across accounts, which I understand was confirmed by examining spot instance pricing between accounts.

UPDATE 6/20/19

AWS added AZ IDs in January 2019 to coincide with the launch of VPC resource sharing. The AZ ID is a consistent identifier that can be used to uniquely identify an AZ.