Quick: How many 9s are in your SLA?

I recently attended an event where one of the speakers was the CTO of a company built on top of Amazon cloud services, the most critical of these being the Simple Storage Service known as Amazon S3.

The S3 service runs “out there” (in the cloud) and provides a scalable repository for applications to store and manage data files. The service can support files of any size, as well as any quantity. So you can put as much stuff up there as you want – and since it is a pay-as-you-go service, you pay for what you use. The S3 service is very popular. An example of a well-known customer, according to Wikipedia, is SmugMug:

Photo hosting service SmugMug has used S3 since April 2006. They experienced a number of initial outages and slowdowns, but after one year they described it as being “considerably more reliable than our own internal storage” and claimed to have saved almost $1 million in storage costs.

Good stuff.

Of course, Amazon isn’t the only cloud vendor with such an offering. Google offers Google Storage, and Microsoft offers Windows Azure Blob Storage; both offer features and capabilities very similar to those of S3. While Amazon was the first to market, all three services are now mature, and all three companies are experts at building internet-scale systems and high-volume data storage platforms.

As I mentioned above, S3 came up during a talk I attended. The speaker – CTO of a company built entirely on Amazon services – twice touted S3’s incredibly strong Service Level Agreement (SLA). He said this was both a competitive differentiator for his company, and also a competitive differentiator for Amazon versus other cloud vendors.

Pause and think for a moment – any idea? – What is the SLA for S3? How about Google Storage? How about Windows Azure Blob Storage?

Before I give away the answer, let me remind you that a Service Level Agreement (SLA) is a written policy offered by the service provider (Amazon, Google, and Microsoft in this case) that describes the level of service being offered, how it is measured, and consequences if it is not met. Usually, the “level of service” part relates to uptime and is measured in “nines” as in 99.9% (“three nines”) and so forth. More nines is better, in general – and wikipedia offers a handy chart translating the number of nines into aggregate downtime/unavailability. (More generally, an SLA also deals with other factors – like refunds to customers if expectations are not met, what speed to expect, limitations, and more. I will focus only on the “nines” here.)

So… back to the question… For S3 and equivalent services from other vendors, how many nines are in the Amazon, Google, and Microsoft SLAs? The speaker at the talk said that S3 had an uptime SLA with 11 9s. Let me say that again – eleven nines – or 99.999999999% uptime. half-an-eye-blinkIf you attempt to look this up in the chart mentioned above, you will find this number is literally “off the chart” – the chart doesn’t go past six nines! But my back-of-the-envelope calculation says it amounts to – on average – less than 32 milliseconds of downtime per year. This is about half what “a blink of your eye” would take – yes, a mere half of an eye-blink. (Which ends with your eyes closed. :-))

This is an impressive number! If only it was true. It turns out the real SLA for Amazon S3 has exactly as many nines as the SLA for Windows Azure Blob Storage and the SLA for Google Storage: they are all 99.9%.

Storage SLAs for Amazon, Google, and Microsoft all have exactly the same number of nines: they are all 99.9%. That’s three nines.

I am not picking on the CTO I heard gushing about the (non-existant) eleven-nines SLA. (In fact, his or her identity is irrelevent to the overall discussion here.) The more interesting part to me is the impressive reality distortion field around Amazon and its platform’s capabilities. The CTO I heard speak got it wrong, but this is not the first time it was misinterpreted as an SLA, and it won’t be the last.

I tracked down the origin of the eleven nines. Amazon CTO Werners Vogels mentions in a blog post that the S3 service is “design[ed]” for “99.999999999% durability” – choosing his words carefully. Consistent with Vogels’ language is the following Amazon FAQ on the same topic:

Q: How durable is Amazon S3? Amazon S3 is designed to provide 99.999999999% durability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.000000001% of objects. For example, if you store 10,000 objects with Amazon S3, you can on average expect to incur a loss of a single object once every 10,000,000 years. In addition, Amazon S3 is designed to sustain the concurrent loss of data in two facilities.

First of all, these mentions are a comment on a blog and an item in an FAQ page; neither is from a company SLA. And second, they both speak to durability of objects – not uptime or availability. And third, also critically, they say “designed” for all those nines – but guarantee nothing of the sort. Even still, it is a bold statement. And good marketing.

It is nice that Amazon can have so much confidence in their S3 design. I did not find a comparable statement about confidence in the design of their compute infrastructure… Reality is that [cloud] services are about more than design and architecture – also about implementation, operations, management, and more. To have any hope, architecture and design need to be solid, of course, but alone they cannot prevent a general service outage which could take your site down with it (and even still lose data occasionally). Some others on the interwebs are skeptical as I am, not just of Amazon, but anyone claiming too many nines.

How about the actual 99.9% “three-nines” SLA? Be careful in your expectations. As a wise man once told me, there’s a reason they are called Service Level Agreements, rather than Service Level Guarantees. There are no guarantees here.

This isn’t to pick on Amazon – other vendors have had – and will have – interruptions in service. For most companies, the cloud will still be the most cost-effective and reliable way to host your applications; few companies can compete with the big platform cloud vendors for expertise, focus, reliability, security, economies-of-scale, and efficiency. It is only a matter of time before you are there. Today, your competitors (known and unknown) are moving there already. As a wise man once told me (citing Crossing the Chasm), the innovators and early adoptors are those companies willing to trade off risk for competitive advantage. You saw it here first: this Internet thing is going to stick around for a while. Yes, and cloud services will just make too much sense to ignore. You will be on the cloud; it is only a matter of where you’ll be on the curve.

Back to all those nines… Of course, Amazon has done nothing wrong here. I see nothing inaccurate or deceptive in their documentation. But those of us in the community need to pay closer attention to what is really being described.  So here’s a small favor I ask of this technology community I am part of: Let’s please do our homework so that when we discuss and compare the cloud platforms – on blogs, when giving talks, or chatting 1:1 – we can at least keep the discussions based on facts.


9 thoughts on “Quick: How many 9s are in your SLA?

  1. Bill Wilder Post author

    Thanks for the comment, Marshall. It really underscores how SLAs are far from guarantees.

    Also – for those not paying as much attention as Marshall – note that the 99.95 SLA he cites is not the SLA for S3 (the main subject of this post). The SLA for Amazon S3 is 99.9.

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  4. Craig

    Just a good discussion about this at work. I was trying to understand the difference between the SLA and the perception of eleven 9s. And now I know. It’s nice to know what they “design” for, but it seems to be different to what they will put their balls on the line for ! :-0

  5. Bhavesh Goswami

    You have slight misconception here. S3 durability guarantee is 99.999999999 not availability.. both are different… Uptime is 99.99%. (As a previous member of the S3 team and now a AWS consulting company CEO, surprised people are still confused with both)

  6. Bill Wilder Post author

    Thanks Bhavesh. You are correct, the *actual* SLA is for uptime. And that was part of the point of this post – we need to be clear about what it is we are talking about in SLAs. But it was the other person mentioned at beginning of this article (CTO of a company running on AWS) who confused the SLAs. And there is no “S3 durability guarantee” – they only mention that’s what they designed for, but not backed. Thanks for the comment!

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