44 lines
2.2 KiB
Markdown
44 lines
2.2 KiB
Markdown
# The Reality of Adding Nines to Your SLOs
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- **期号**: SRE Weekly Issue #432（2024-07-07）
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- **作者**: Thomas Stringer
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- **链接**: https://trstringer.com/slo-adding-nines/
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## 简介
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A concise breakdown of the math involved in getting that extra nine of reliability.
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> It all boils down to creating the SLOs and requirements to keep your users happy, but nothing more. Unnecessary reliability is a high cost.
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## 正文
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A lot of times there is an incorrect assumption that adding nines to your SLO is linear, both in gains and associated costs. Unfortunately, this idea can cause a lot of unnecessary burden on the engineers that have to build out and maintain the solution.
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I’m a big fan of looking at the data, and doing the math.
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**Downtime in 30 day SLO window**
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| Uptime | Nines | Downtime (minutes) |
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|---|---|---|
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| 99% | 2 | 432 |
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| 99.9% | 3 | 43 |
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| 99.99% | 4 | 4 |
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| 99.999% | 5 | 0.4 |
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| 99.9999% | 6 | 0.04 |
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Going from 2 nines to 3 nines, we get a whopping gain of a little less than 400 minutes uptime. This is almost 6 and a half hours of uptime in a month, just a little shy of a full business working day. I’d say that’s a pretty significant leap in uptime and reliability!
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But going from 4 nines to 5 nines gives us only about a 3 minute uptime gain. Will your users even care? Will they even notice? It’s helpful to visualize the gains as you start climbing the ladder of nines:
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For a lot of business stakeholders, this probably isn’t what they thought it would be. Much like the uptime gains, the added costs and complexity are also very much not linear:
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When you start adding nines, especially beyond 3 nines, it because dramatically expensive and complex. Having less than 5 minutes of downtime over the course of a month requires a high level of redundancy, distribution, and automation. You can no longer rely on the on-call engineer to respond and mitigate to meet this SLO.
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It all boils down to creating the SLOs and requirements to keep your users happy, but nothing more. Unnecessary reliability is a high cost.
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