What is Availability Sets?

• An availability set is a logical grouping of VMs to provide for redundancy and availability
• “Availability Set” refers to two or more Virtual Machines deployed across different Fault Domains to avoid a single point of failure.
• There is no cost for the Availability Set itself, you only pay for each VM instance that you add to availability set
• If two or more VMs should be provisioned within an availability set, SLA will be 99.95%

Concepts

• Fault Domain:
  • A collection of servers that share common resources such as power and network connectivity
  • A fault domain is factually a rack of the servers which consumes mostly subsystem like network, power, cooling, etc.
• Update Domain:
  • A group of VMs and underlaying hardware that can be rebooted together
  • Think update domain as “a server” (hardware + hardware manager + service febric + VMs) in rack
  • For planned maitenance, hardware or software level patch/update, each update domain will be shutdown one at a time
• Each virtual machine in the Availability Set is assigned an Update domain and Fault domain by the Azure platform
• VMs in Availability Set are automatically distributed across multiple fault domains
• You can’t add an existing VM to an availability set after it’s created, you can only provision/add new VMs to Availability Set. So, plan accordingly from the beginning
• Details: https://docs.microsoft.com/en-us/azure/virtual-machines/availability-set-overview#how-do-availability-sets-work

Purpose of Availability Set

• Each virtual machine in the Availability Set is assigned an Update domain and Fault domain by the Azure platform
• Availability Sets ensure that the Azure virtual machines are deployed across multiple isolated hardware nodes in a cluster
• Availability Set prevents single point of failure
  • If a server in a rack (update domain) goes down or a whole rack (fault domain) goes down, your VMs are safe because Availability Set ensures that VMs are spread across “multiple fault domains” (“multiple racks”)
  • For planned maitenance (i.e. patching by Microsoft), one update domains (servers) will be rebooted one at a time, and since VMs are spread across multiple racks, VMs are safe during planned maintenance
• SLA
  • Availability Set gives 99.95%
  • The reason SLA is not five 9’s (99.999%) is that, in case of Datacenter level failure or Region level failure, your VMs are not safe

Creating Availability Set

• Go to: https://portal.azure.com/#create/Microsoft.AvailabilitySet-ARM
• Fillup details
• Create + Review > Create
• Links:
  • Windows: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/tutorial-availability-sets
  • Linux: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/tutorial-availability-sets

Adding VM to Availability Set

• While creating VM, there is “Availability Option” in the basic tab
• You can create Availability Set beforehand or can create new Availability Set while creating VM