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I don't see how transparently replicated storage could be implemented without sharing something.

The transparent replication is inside the availability zone; as I understand it, Amazon doesn't provide any sort of user visible direct sharing between multiple AZs, e.g. to copy or move an EBS you have to snapshot it first ... which was of course a control API they blocked during much of this mess.

In which case they shouldn't replicate across availability zones.

'shared nothing' is the only way to islandize failures.

And that's why they have multiple fully-isolated regions. Availability zones are a purposeful tradeoff that provides easier to use service with higher inter-zone communication performance and lower cost.

No, that's not what Amazon says.

The following is from the AWS web site [1]:

> Availability Zones are distinct locations that are engineered to be insulated from failures in other Availability Zones and provide inexpensive, low latency network connectivity to other Availability Zones in the same Region. By launching instances in separate Availability Zones, you can protect your applications from failure of a single location.

No mention of tradeoffs.

[1] http://aws.amazon.com/ec2/

The tradeoff is in that AZs are "engineered to be insulated" as opposed to being actually, or naturally isolated. Prior to their downtime, I've had plenty of conversations with folks that I work with about AWS and we've always assumed that AZs are not 100% isolated. I can see how someone can read "engineered to be insulated" the other way, but I generally read these kinds of materials as guaranteeing nothing beyond the most limited possible reading, and probably not even that.

The quoted statement doesn't say that isolation is 100% or that multiple AZs can't ever ever fail at the same time. It says that if only one AZ goes down and you have servers in another, then those servers will still be up, which should be obvious. Insulated doesn't even mean the same thing as isolated.

Sure, I don't disagree with what you are saying. However I think that the way Amazon presents the concept of an AZ is that it IS isolated from other AZ's in the same region.

Even the name, 'Availability Zone' implies that it is isolated from other 'Availability Zones' in the same region. And that text I quoted does nothing but substantiate that inference.

I just think that Amazon are misleading here. Maybe they shouldn't call it an Availability Zone.

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