Azure VM - Update Domain and Fault Domain in Avaibility set RRS feed

  • Question

  • I am reading about availability set in Azure Virtual Machine. And it seems a bit confusing to me.

    I have few questions, would appreciate if someone can answer that. 

     1. Microsoft document says with two or more machines in availability set gives 99.95% availability. if this is the case why they have 3 maximum Fault Domain and 20 maximum Update domain. if I choose a max of both would I get more availability than 99.95%? if not, what is the purpose of having more updates or fault domains than 2? 
     2. If I have 3 fault domain and 20 update domain, how many physical machines will be created? 20 max(update_domain, fault_domain) or 23 (update_domain + fault_domain) 
     3. can it be possible to have less number of update domain than fault domain? i.e. 2? 
    Wednesday, March 18, 2020 3:14 AM

All replies

  • Hi,

    I will answer your questions one by one.

    Answer 1:

    When we deploy two or more machines in availability set azure promises availability equal to or more than 99.95%.

    Azure provides a fixed number there, so that if the vms in the particular availablity set has availablity less than the quoted value, Appropriate compensation will be provided.

    When we use more VM's and select more fault and update domains, The chance of a one machine going down due to any issue(azure issue or customer issue) is less. Lets say we are deploying 10 VM's in a Availability set with 3 fault domains and 10 update domains, Our vms will be spread across the datacenter. So the chance of vms going down is less . we can expect more availability than the specified value.  

    Answer 2:

    Instead of how many physical servers, we can take as how many physical server groups.

    While datacentre will be split into 3 parts and each part will go into a fault domain.  Each fault domain will have separate infra like storage network, power etc.  So if one of the fault domain goes down, It wont affect the vms in other fault domains.

     Same way whole datacentre will be split into 20 logical portions and each one will be called as one update domain.
    At a time only one portion will be updated with new softwares.  

    So we need to choose update and fault domain separately.  Both server their purpose.

    Answer 3:

    Since fault domains have physical isolation, it will be 2 or 3 in Azure regions. Update domains will be around 20.

    You can have less number of update domains than fault domains. But thats not recommended because when the count decreases, the chances of vms going down from the same domain increases.

    Wednesday, March 18, 2020 12:23 PM