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Windows Azure periodically updates the operating system that hosts the instances of an application. A virtual machine is shut down when an update is applied. An update domain is used to ensure that not all of the virtual machine instances are updated at the same time. When you assign multiple virtual machines to an availability set, Windows Azure ensures that the machines are assigned to different update domains.
You should use a combination of availability sets and load-balancing endpoints to help ensure that your application is always available and running efficiently.
Configuring Availability Sets
You should ALWAYS specify an availability set when creating more than one virtual machine for the same purpose.
Typical Examples include :
- Two or more web servers
- Two or more Database servers
- Two or more AD servers
Specifying an availability set in these situations gives you multiple advantages.
Highly Available Hardware
Having two or more VMs in availability sets guarantees that that VMs are spread across multiple racks in the Windows Azure Data Centers. This means redundant power supply, switches and servers.
Rolling Host Updates
Grouping VMs in availability sets also gives the Windows Azure Fabric Controller the information it needs to intelligently update the host OSs that your guest VMs are running on. Without availability sets the FC would have no idea that two machines were serving the same purpose and could reasonable take them both down for host OS updates.
99.95% SLA
If you want to ensure the 99.95% SLA guaranteed by Windows Azure for uptime. Availability sets is the way to go.
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