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Imagine I have two EC2 instances A and B, in the same region/vpc, but in different AZs.

I want to transfer 1TB from A to B.

Now let's imagine two scenarios:

  1. I send the data from A to B directly.
  2. I upload the data from A to S3, then download the data from S3 to B, and finally delete the data from S3.

Pricing-wise:

  1. I get charged at $.01/GB twice (once when data gets out of A, once getting into B), so 1TB * $.01/GB * 2 = $20
  2. Traffic between EC2 and S3 within a region is free, so I get charged, say for storing 1TB for a day. 1TB/day = 33GB/Month. Taking $0.023 as our per-GB-Mo price, that gives us, 33 * $0.023 = $.759.

Is it actually the case that going through S3 is (much, much) cheaper than going direct (even when storing the data for a whole day rather than an hour) or is my analysis flawed?

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    Storing 1TB for a month costs about US$23, so storing it for a day costs (23 / 30 = ) $0.78. The S3 pricing page says you pay $0.098 / GB for all data transferred out of S3, though I suspect it's zero rated for VPCs the pricing page doesn't say that, so you should check to be sure.
    – Tim
    Feb 7 at 18:51
  • You're correct, I was off by one order of magnitude for S3, which makes the difference between both approaches even larger. it is my understanding that ec2 <-> s3 traffic within the same region, without going over the internet is indeed free
    – foo
    Feb 8 at 10:56
  • I think EC2 to S3 traffic is zero rated. It could be the situation where the S3 service has retrieval fees separate from bandwidth - I'm not saying that's the case but it's worth considering. The AWS pricing is fairly complex.
    – Tim
    Feb 8 at 18:52

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