> all AZs are within the same physical facility
The TL;DR from AWS documentation [1]:
An Availability Zone is represented by a region code followed by a letter identifier; for example, us-east-1a. To ensure that resources are distributed across the Availability Zones for a region, we independently map Availability Zones to identifiers for each account. For example, your Availability Zone us-east-1a might not be the same location as us-east-1a for another account. There's no way for you to coordinate Availability Zones between accounts.
The long and confusing explanation:
At least not in us-east-1 and us-west-1-2, but I am pretty sure many of the large regions are also run in multiple physical facilities.
The so-called availability zone is an abstract and virtual concept. Let us use us-east-1 as an example.
Assume the following:
* Physical DC buildings: Queens, Brooklyn, Manhattan, Staten Island
* AWS accounts: Joe, Alice, Bob
* AZ: us-east-1a, us-east-1b, us-east-1c, and us-east-1d
Every AWS account in us-east-1 region is assigned three AZs. But for sake of this explanation, we assume only two.
* Joe: 1a, 1b
* Alice: 1a, 1b
* Bob: 1a, 1c
You now ask, "WTF?" but you let this go, think this is done for capacity reason. So do we actually have four different physical facilities, one per each AZ? Nope.
So is 1a and 1b in the same facility? Not necessarily, but very possible.
So 1a and 1b in Queens, 1c in Brooklyn, and 1d in Manhattan? Nope.
So what the fuck is AZ? What is the relationship between AZ and physical facility?
Think about virtual memory address space.
Joe's 1a and Bob's 1a are in Queens, but Alice's 1a is in Manhattan. But Joe's 1a and Bob's 1a are on a different floor, different racks, while Joe's 1b and Bob's 1c are in Brooklyn and on the same floor. This is why certain customers run out m3.xlarge in 1a but others don't in their 1a.
In essence, AZ is a label and is unique per account. AZ is very similar how virtual memory address in OS looks like.
We learned this because our EMR failed due to low capacity in one account.
[1]: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-reg...