Availability Matrix
High Availability (HA) is a measure of system resilience. Understanding your 'Error Budget' is the first step in SRE (Site Reliability Engineering).
Availability (SLA) Matrix
Reliability Engineering Downtime Budgeting
Availability is not just about failure rate. It is defined as MTBF / (MTBF + MTTR). You can increase availability either by increasing the Mean Time Between Failures or by decreasing the Mean Time To Repair (e.g., keeping cold spares on site).
The Math of "Nines"
In professional infrastructure, availability is often expressed in the number of 'nines'. Moving from 99.9% (Three Nines) to 99.999% (Five Nines) is not just a 1% improvement—it is a 100x reduction in allowed downtime.
Downtime Budget Table (Monthly):
- 99.9%: 43 minutes allowed.
- 99.99%: 4 minutes 23 seconds allowed.
- 99.999%: 26 seconds allowed.
Achieving "Five Nines" requires near-instantaneous automated failover. At this level, human intervention is often too slow to stay within the error budget, necessitating advanced orchestration and redundant hardware paths.
Technical Standards & References
Related Engineering Resources
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