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.
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
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.
Quite. South Africa has a string of rolling black-outs though...some lasting as long as 8 hours at a time during bad spells. Some you'll be running off generators as primary source (with no backup) far more than you'd normally expect even if you have multiple data centres.
Easiest way around that is to locate it in an area exempt from the rolling blackouts as I said.
There hasn't been an 8 hour blackout in Cape Town due to loadshedding. The longest was 4 hrs I believe. They have a plan to get this sorted out in a year and a half, in the meantime DC's will just have to rely on their power redundancy (which has to be in place anyway).
That means 3 data centres.
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