I have seen some Azure VM downtime concerns from Windows Azure users who have minimum 2 or more instances to meet to 99.95% SLA. The specific concerns are related with VM downtime when Guest and Host OS update is scheduled.
So lets consider following 3 scenarios:
Note: In Scenario 2 and 3 you have some control to decide when and how all of your instances will be updated while Guest OS is being updated. And in this scenario, the concept of upgrade-domains is used when the Guest OS update is performed.
Guest OS Update (OS in your Azure VM):
To master the art of setting Upgrade Domain/ Fault domain, I would suggest you to read the blog below and architect you multi-instance Windows Azure application after digesting this info:
Host OS Update (OS in which you Azure VM is running):
- You don’t have any control when and how Host OS will be updated.
At last, now you can have better idea that you can control the timing of you own VM (web/worker role) OS updates using upgrade domain/fault domain concept up to certain extent, but you cannot control when the host OS is updated.
Now you may ask how often host OS gets updated usually? Monthly/Weekly/Daily? Also does it tend to be scheduled during off-peak/night time of the timezone which the to-be-updated datacenter is located in, or it can just happen at any time during the day?
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