How to calculate overall availability given individual object availability percentages?

Example 1: I have sql server with FCI.

VM1 has 99.9% availability

VM2 has 99.9% availability

What is total availability calculation?

Example 2: same as above plus disk storage has 99.9% availability. So what is total availability?

Example 3: Suppose I have 1 VM and 1 disk storage. Each with 99.9% availability. So what is total availability calculation?

CodePudding user response：

AWS is using Calculating Total System Availability article for availability calculation. So in your setup you have to recognize what is parallel and what is serial relationships. For basic scenarios:

Serial Configuration

A(VM1   disk) = 0.999 * 0.999 = 0.998

Parallel Configuration

Assuming VM2 is a redundancy for VM1

A(VM1, VM2) = 1 - (1-0.998)(1-0.998) = 0.99996

CodePudding user response：

Example 1: If you have 2 VMs and each one has 99.9% availability and your total availability like below

.999 * .999 = .998 it means monthly 1h 27m 39s of downtime.

Example 2: If you have 2 VMs and each one has 99.9% availability and one more disk storage has 99.9% availability. your total availability like below

.999 * .999* .999=.997 it means monthly 2h 11m 29s of downtime.

Example 3: if you have 1 VM and 1 disk storage. Each with 99.9% availability and your total availability like below

.999* .999=.998 it means monthly 1h 27m 39s of downtime.

The SLA for Google cloud VMs when you have two or more VMs configured across various availability zones is 99.99%.

You can also use this link for compute engine and storage availability and use link for SLA calculations,