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Web server: us-east-1a DB Server us-east-1b

The ping between them is under 2ms so I'm guessing it's fine but I wanted to check.

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You should be fine. The additional latency is negligible. I would recommend setting up your slave server in a different AZ from you master.

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From the EC2 site:

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.

Since they are both in us-east region Amazon's probably trying to make their "low-latency" promise real.

But what TomTOm said is totally true, depends on what you're doing. If this type of 2ms latency is your bottleneck you'll probably care.

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Depends what you do. Seriously. Also the traffic I think may cost. I work mostly on databases where "10g connection, same room" is a requirement for good performance. It really depends what you do.

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My understanding is that inter-AZ traffic is free for AWS. Inter-region costs. – ceejayoz May 9 '11 at 20:09
Free? Like what? No money - great. But the OP says 2ms latency and you may find out that this COSTS you. PERFORMANCE is also called cost in IT. 2ms is a lot more than in one room. So, if you do tons of round trips this has a HUGH cost in terms of adding latency like hell. You can gladly say this isn not cost when your site goes to a crawl due to many sql statements and adding needless latency. – TomTom Dec 6 '11 at 9:24

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