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Microsoft is adding a new feature that allows you to control the forced outages that occur to virtual machines when patches are delivered to Azure’s compute hosts.
Ideally, when you deploy a service using Azure virtual machines, those virtual machines should be a part of a valid availability set. Here’s a quick reminder on availability sets:
There is no live migration in Azure. Imagine tens of thousands of machines live migrating on a single cluster and what that would do to the infrastructure and the applications in the virtual machines! Instead, when the host reboots the virtual machines have downtime.
An availability set tags virtual machines so that Azure knows to put them into different update domains. When Microsoft deploys updates to Azure, they do so in an ordered fashion, one update domain at a time. This means that only a small number of hosts are ever offline because of patching and rebooting. If you have configured anti-affinity by using availability sets, then only one (or a few) virtual machines will ever be down at one time.
The key part is: this must be a valid availability set to achieve the 99.95 percent SLA for the service on those machines. Putting one domain controller and one file server into an availability set achieves nothing for uptime and the SLA won’t apply. But putting 2 load balanced web servers into an availability set qualifies the web service for the SLA and minimizes downtime
An Illustration of Update Domains and Fault Domains in Azure [Image Credit: Microsoft]
The entire process takes between 15-30 seconds. Most of us never notice that brief amount of downtime, however:
To be honest, there’s not much to this feature from our point of view but it will be very valuable to customers that must only have downtime during maintenance time of their own choosing. The new Planned Maintenance feature for Azure virtual machines allows that to happen:
Relocating Azure Virtual Machines Using Service Health – Planned Maintenance [Image Credit: Microsoft]
This is a simple feature and but it should save a few scalps in the operations departments of many Azure customers.
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