Summary: EC2 availability zone names in different accounts do not match to the same underlying physical infrastructure. This article explains a trick which can be used to figure out how to match availability zone names between different accounts.
As of the updating of this article (2011-12-23) Amazon EC2 (Elastic Compute Cloud) has twenty different availability zones in eight regions. A region can be thought of as a specific area of the world. An availability zone can be thought of roughly as a data center, defined such that no single failure scenario should affect two availability zones.
The current regions are:
us-east-1- East coast of the United States (Northern Virginia)
us-west-1- West coast of the United States (Northern California)
us-west-2- West coast of the United States (Oregon)
sa-east-1- East coast of South America (São Paulo, Brazil)
eu-west-1- Western Europe (Dublin, Ireland)
ap-southeast-1- SouthEast Asia (Singapore)
ap-northeast-1- NorthEast Asia (Tokyo)
- GovCloud - US Government use only
The availability zones in those regions are given the region name plus simple letters appended. For example:
When you start EC2 instances, you can specify an availability zone or let Amazon pick one for you. You always have a default region which can be overridden when starting instances.
In order to prevent an overloading of a single availability zone when everybody tries to run their instances in
us-east-1a, Amazon has added a layer of indirection so that each account’s availability zones can map to different physical data center equivalents.
For example, zone
us-east-1a in your account might be the same as zone
us-east-1c in my account and
us-east-1d in a third person’s account.
In fact, given the way that Amazon has set this up, I would not be surprised if Amazon may not occasionally reassign availability zone names which you are not currently using. For example, Amazon added the fourth availability zone in the
us-east-1 region, but I suspect this might not be
us-east-1d in all accounts (especially new ones).
On occasion, users sometimes want to know if instances in different accounts are running in the same availability zone. Or, users might want to know which availability zone is the one with which people are currently experiencing a particular problem.
You’ll often see users say that there is a problem in zone
us-east-1a but this isn’t very helpful for other users because (as described above) that name only has significance within the original user’s account.
What would be helpful is a unique identifier which maps to the underlying physical infrastructure (e.g., data center) and can be mapped to the different availability zone names in each account.
I believe that Amazon may have inadvertently let slip a way to obtain this in the current implementation of the reserved instance offering ids. In my experiments so far, these seem to be tied to something outside of the account’s availability zones and, though the ids are the same, they are mapped to different availability zone names for different accounts.
To demonstrate this I’ve arbitrarily chosen the reserved instance offerings for m1.small, one year, Linux/UNIX.
To list the mappings for a single account, you can use a command like:
ec2-describe-regions | cut -f2 | while read region; do ec2-describe-reserved-instances-offerings --region $region | perl -ne 'print "$2 $1\n" if m%\S+\t(\S+)\t(\S+)\tm1.small\t1y.\t.*\tLinux/UNIX(\t(.(?!Utilization)|Medium Utilization)*)$%' done | sort
Here are the mappings for one of my accounts (let’s call it “Blue”):
ap-northeast-1a 4b2293b4-ed86-4e9c-a41d-b9aa6ab6d143 ap-northeast-1b ceb6a579-a3ba-4488-ae42-59ca92a3d26f ap-southeast-1a d586503b-ce55-4e1e-8012-f87395ccfb39 ap-southeast-1b 649fd0c8-66cb-4867-a124-70e7b38902b1 eu-west-1a c48ab04c-0bd0-4be9-8db5-a4bad61c6c58 eu-west-1b d586503b-7025-44e8-8487-09907b6b0e7e eu-west-1c c48ab04c-ca5b-43be-999b-ed45d67a058e sa-east-1a ceb6a579-ef87-4741-bbb5-f52911e182b4 sa-east-1b 248e7b75-96bb-4161-b50a-2f042094f022 us-east-1a 438012d3-80c7-42c6-9396-a209c58607f9 us-east-1b 60dcfab3-06bb-4b68-9503-53bf89823b5e us-east-1c ceb6a579-757c-474b-b09b-52c84b605767 us-east-1d 649fd0c8-5d76-4881-a522-fe5224c10fcc us-west-1a c48ab04c-446f-416d-b7e1-75db17b1354c us-west-1b e5a2ff3b-79b4-4217-8c93-ebf1d633dd6e us-west-1c 4b2293b4-def5-468b-ae97-63886afe7a09 us-west-2a 60dcfab3-d0b1-48f5-bb2a-031bba5ec5e5 us-west-2b ceb6a579-9c3b-4098-b745-c3e256d612c5
Here are the mappings for a different account (let’s call it “Red”):
ap-northeast-1a 4b2293b4-ed86-4e9c-a41d-b9aa6ab6d143 ap-northeast-1b ceb6a579-a3ba-4488-ae42-59ca92a3d26f ap-southeast-1a 649fd0c8-66cb-4867-a124-70e7b38902b1 ap-southeast-1b d586503b-ce55-4e1e-8012-f87395ccfb39 eu-west-1a d586503b-7025-44e8-8487-09907b6b0e7e eu-west-1b c48ab04c-0bd0-4be9-8db5-a4bad61c6c58 eu-west-1c c48ab04c-ca5b-43be-999b-ed45d67a058e sa-east-1a 248e7b75-96bb-4161-b50a-2f042094f022 sa-east-1b ceb6a579-ef87-4741-bbb5-f52911e182b4 us-east-1a ceb6a579-757c-474b-b09b-52c84b605767 us-east-1b 438012d3-80c7-42c6-9396-a209c58607f9 us-east-1c 60dcfab3-06bb-4b68-9503-53bf89823b5e us-east-1d 649fd0c8-5d76-4881-a522-fe5224c10fcc us-west-1a e5a2ff3b-79b4-4217-8c93-ebf1d633dd6e us-west-1b c48ab04c-446f-416d-b7e1-75db17b1354c us-west-1c 4b2293b4-def5-468b-ae97-63886afe7a09 us-west-2a 60dcfab3-d0b1-48f5-bb2a-031bba5ec5e5 us-west-2b ceb6a579-9c3b-4098-b745-c3e256d612c5
From this, I theorize that availability zone
us-east-1a in account Blue is the same as availability zone
us-east-1b in account Red, but availability zones
us-east-1d happen to be the same in both accounts.
Please note that this approach is not a documented feature of Amazon EC2. I may be misinterpreting what I am seeing and the mappings may be completely random for different accounts.
Amazon could at any time restructure how these values work so that the described offering ids cannot be used between accounts or do not map to any common infrastructure.
Use at your own risk and please post a comment if you find out any further data to support or disprove this theory.
[Update 2011-04-25: Tweaked command line to exclude more instance types Amazon has added. Updated info for current ids in current regions/zones.] [Update 2011-12-23: Tweaked command line to exclude more instance types Amazon has added. Updated info for current ids in current regions/zones.]