1. Mistake: treating zero downtime as a default. An availability zone is an isolated physical location within a cloud region; distributing infrastructure across zones can improve resilience, but the required level must justify its cost. section 13.2
Four nines in Azure
For four nines of availability, an Azure IaaS design requires two virtual machines split across two availability zones. Azure SQL Database can instead use the Business Critical or Premium tier. section 13.2
Table 13.1 Levels of availability (view table figure)
| Level of availability |
Downtime per week |
Downtime per month |
|---|---|---|
| 99% |
1 hour, 40 minutes, 48 seconds |
7 hours, 18 minutes, 17 seconds |
| 99.9% |
10 minutes, 4 seconds |
43 minutes, 49 seconds |
| 99.99% |
1 minute |
4 minutes, 23 seconds |
| 99.999% |
6 seconds |
26 seconds |
Compare availability targets with their allowed downtime before choosing a multi-zone or other high-availability design. section 13.2
Use this checklist before configuring a topology:
- Clarify the application’s availability requirements. section 13.2
- Compare the financial cost of downtime with the cost of the high-availability or disaster-recovery solution. section 13.2
