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5.2. VM Availability (SLA, Availability Sets, Availability Zones).md

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VM Availability

  • Microsoft Azure provides a Service Level Agreement (SLA)
    • backed by a financial service credit payment for IaaS Virtual Machines.
    • Depends on the deployment of the virtual machine and what resources it uses.

Availability Set

  • Ensures SLA can be provided.
    • One VM being available at least 99.95% of the time.
  • Ensures VMs you deploy within an Azure data center are isolated from each other.
  • Ensures that all virtual machines that are added to the set are placed in such a way as to ensure that neither hardware faults or Azure fabric updates that is unplanned and planned maintenance events can bring down all of the virtual machines.
  • Application availability can be impacted by:
    • Unplanned hardware maintenance event
    • An unexpected downtime
    • Planned maintenance events
  • 💡 To reduce or remove the impact of downtime:
    • Place virtual machines in an availability set for redundancy.
    • Use managed disks for all VMs placed in an availability set.
    • Use Scheduled Events to respond to events.
    • Place each tier of your application in a separate availability set.
    • Use a load balancer in combination with availability sets.
  • 💡 Avoid single instance VMs in an availability set.
    • They are subject to any SLA unless all the Operating System and Data disks are using Premium storage.

Update and Fault Domains

  • Each machine in the Availability set is placed in an Update Domain and a Fault domain.
  • A Fault Domain (FD) is essentially a rack of servers.
    • It consumes subsystems like network, power, cooling etc.
  • Update Domain (UD)
    • Purposeful move to take down one (or more) of your servers.
    • It will walk through your update domains one after the other.
  • 📝 FDs come in sets of 2 and UDs come in sets of 5 (default)
    • So if you deploy more than 5 VMs in an availability set they'll end up in same UD and FD.

Multiple availability sets

  • E.g. N-tier availability sets
    • An extension of the availability set model is used logically to place individual tiers of an application into separate Availability Sets.
    • E.g. put front-ends in one, and data tier in another availability set.

Availability Zones

  • Advent of a data center-wide fault would prevent the Availability set from functioning.
  • Allows for a complete data center failure and keep your VM based application running.
    • Zone = separate zone or building within a single Azure region.
  • You can set the count of zones while creating VM.
    • There is a maximum of three Availability Zones per supported Azure region.
    • Each Zone operates on an entirely isolated power source, cooling system, and network infrastructure.