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A common misconception, but no, AWS AZs are not DCs. A single AZ may be composed of multiple data centers[1], and a region may incorporate facilities that do no

A common misconception, but no, AWS AZs are not DCs. A single AZ may be composed of multiple data centers[1], and a region may incorporate facilities that do not serve a public AZ[2], or that supply other capabilities[3]. [1] They'd be necessarily close together due to speed-of-light constraints. [2] You may infer this from S3's triple-zone replication, which is still somehow magically fulfilled in regions that only have two public AZs. [3] most obviously, Direct Connect.

SideburnsOfDoom 7y ago
You're right, I should have said "at least three data centers" It was in response to parent comment saying "the datacenter", which seems to be assuming that there was only one. But there will be multiple.
mjlee 7y ago
To clarify, it means at least 3 DCs per AWS documentation. https://aws.amazon.com/about-aws/global-infrastructure/regions_az/ https://aws.amazon.com/about-aws/global-infrastructure/regio...
- darkwater 7y ago
  Oh, very nice link, thanks! > AZ’s are physically separated by a meaningful distance, many kilometers, from any other AZ, although all are within 100 km (60 miles) of each other. This answers my previous question I guess EDIT: well, actually, no. I wanted to know distance between DC in the same AZ :)
  - EwanToo 7y ago
    If you really want to know, you can look it up on wikileaks https://wikileaks.org/amazon-atlas/map/ https://wikileaks.org/amazon-atlas/map/
darkwater 7y ago
Do you happen to know how physically far can be AZs in the same region? The EBS sticking to the same AZ hints that they may be not that close, but I'm also very surprised to discover that 1 AZ != 1 DC
- dgemm 7y ago
  Most AZs are 1 DC, but some are too large to fit in a single building. I don't think there is a distance limit between AZs but instead a latency limit. James Hamilton covered it in one of his keynotes a few years ago. EBS sticking in the same AZ has nothing to do with distance and everything to do with keeping failures within an AZ
  - SideburnsOfDoom 7y ago
    > I don't think there is a distance limit between AZs but instead a latency limit Ok, fleshing that out via google: "The connection between AZs is expected to typically achieve low single-digit millisecond latency between them on average." https://knowledgebase.progress.com/articles/Article/expected-latency-between-aws-availability-zones https://knowledgebase.progress.com/articles/Article/expected... and 1 light-millisecond = 299792.458 meters, so nearly 300 km / 186 miles. https://www.translatorscafe.com/unit-converter/en-US/length/95-1/ https://www.translatorscafe.com/unit-converter/en-US/length/... So in 2 milliseconds, light travels 599km or 372 miles. That's a _hard_ limit. IIRC, modern communications is at a fraction of lightspeed, which would lower it. There's no reason _not_ to have the AZs for a region in the same metro area, so separated by 10s of km, tops. 10km = 0.0333 light-milliseconds.
    - bbgm 7y ago
      You need to take into account the speed of light in fiber optics and switching latency. For fiber you're looking at about 5 µs/km
      - SideburnsOfDoom 7y ago
        > about 5 µs/km Which puts it at 200km in a millisecond (5 microseconds is 1 / 200 of a millisecond.) Like I said "modern communications is at a fraction of lightspeed", although I didn't want to guess what fraction. But 200 / 600 = 1/3 is a reasonable fraction, seems legit.

SideburnsOfDoom 7y ago

You're right, I should have said "at least three data centers" It was in response to parent comment saying "the datacenter", which seems to be assuming that there was only one. But there will be multiple.

mjlee 7y ago

To clarify, it means at least 3 DCs per AWS documentation. https://aws.amazon.com/about-aws/global-infrastructure/regions_az/ https://aws.amazon.com/about-aws/global-infrastructure/regio...

darkwater 7y ago
Oh, very nice link, thanks! > AZ’s are physically separated by a meaningful distance, many kilometers, from any other AZ, although all are within 100 km (60 miles) of each other. This answers my previous question I guess EDIT: well, actually, no. I wanted to know distance between DC in the same AZ :)
- EwanToo 7y ago
  If you really want to know, you can look it up on wikileaks https://wikileaks.org/amazon-atlas/map/ https://wikileaks.org/amazon-atlas/map/

darkwater 7y ago

Do you happen to know how physically far can be AZs in the same region? The EBS sticking to the same AZ hints that they may be not that close, but I'm also very surprised to discover that 1 AZ != 1 DC

dgemm 7y ago
Most AZs are 1 DC, but some are too large to fit in a single building. I don't think there is a distance limit between AZs but instead a latency limit. James Hamilton covered it in one of his keynotes a few years ago. EBS sticking in the same AZ has nothing to do with distance and everything to do with keeping failures within an AZ
- SideburnsOfDoom 7y ago
  > I don't think there is a distance limit between AZs but instead a latency limit Ok, fleshing that out via google: "The connection between AZs is expected to typically achieve low single-digit millisecond latency between them on average." https://knowledgebase.progress.com/articles/Article/expected-latency-between-aws-availability-zones https://knowledgebase.progress.com/articles/Article/expected... and 1 light-millisecond = 299792.458 meters, so nearly 300 km / 186 miles. https://www.translatorscafe.com/unit-converter/en-US/length/95-1/ https://www.translatorscafe.com/unit-converter/en-US/length/... So in 2 milliseconds, light travels 599km or 372 miles. That's a _hard_ limit. IIRC, modern communications is at a fraction of lightspeed, which would lower it. There's no reason _not_ to have the AZs for a region in the same metro area, so separated by 10s of km, tops. 10km = 0.0333 light-milliseconds.
  - bbgm 7y ago
    You need to take into account the speed of light in fiber optics and switching latency. For fiber you're looking at about 5 µs/km
    - SideburnsOfDoom 7y ago
      > about 5 µs/km Which puts it at 200km in a millisecond (5 microseconds is 1 / 200 of a millisecond.) Like I said "modern communications is at a fraction of lightspeed", although I didn't want to guess what fraction. But 200 / 600 = 1/3 is a reasonable fraction, seems legit.