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I'm doing capacity planning for my DR site and i'm trying to understand break-even point for Light/Medium/Heavy reserved instances vs the Ondemand. I have plotted my values on a graph as shown in the picture below:

http://imgur.com/wLAOIX1

Am i correct,in making the following assertions ?

  1. For an m3.large instance running 24/7 (100% utilization),there is no break-even point and does it makes sense to prefer an Ondemand over light RI ?

  2. The break-even point for a High RI over Ondemand is some where around 170 days. Am i right in saying,choosing High RI pays dividends only if,the uptime of that instance is more than 170 days or 4080 Hours (46.44 %) in an year?

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  • If you're running 24/7, it's not "light usage". If you're running less than 46% of a year, it's not "heavy usage". There's a reason they picked these terms.
    – ceejayoz
    Oct 24, 2014 at 12:02
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    You do realise that for most organisations a disaster recovery site is actually mostly idle and only active during a the actual disaster (typically those are of relatively short duration) and during fail-over tests?
    – HBruijn
    Oct 24, 2014 at 12:21
  • @HBruijn I don't believe it's a software license issue ...just an understanding of the Amazon EC2 types.
    – Nathan C
    Oct 24, 2014 at 12:23
  • @NathanC That has been used as the closing reason for a number of other questions that deal with cloud pricing, which IMHO this seems to be a variant of.
    – HBruijn
    Oct 24, 2014 at 13:21
  • @LOL there are a couple of different math and logical errors in your graph. Oct 24, 2014 at 20:25

1 Answer 1

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If you're using it 100% of the time, use heavy utilization for the highest discount rates. However, if it's going to be a disaster recovery solution, you might as well use "light utilization" unless the higher price (for longer periods) and uncertain capacity of on-demand is enough for you.

The major difference is that with on demand, you're not promised capacity, so it can throw your entire DR setup out the window, hence why reserved instances exist. However, the upfront cost is a deterrent...

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