How Many App1 Instances Will Remain Available during Planned Maintenance?

C

Only one update domain is rebooted at a time. Here there are 7 update domain with one VM each (and 13 update domain with no VM).

In an Azure availability set, virtual machines are distributed across multiple fault domains and update domains. Fault domains and update domains are used to ensure that the virtual machines in an availability set are not all affected at the same time by hardware or software maintenance, physical network maintenance, or other localized events.

In this scenario, the number of fault domains is set to 3 and the number of update domains is set to 20. This means that the seven virtual machines running App1 will be evenly distributed across three fault domains and 20 update domains.

During a period of planned maintenance, only one update domain is taken offline at a time. This means that virtual machines running App1 in the offline update domain will be unavailable, while the virtual machines running App1 in the other update domains will remain available.

To determine how many App1 instances will remain available during a period of planned maintenance, we need to calculate the number of virtual machines that are not in the offline update domain. Since there are 20 update domains and only one is offline, there will be 19 update domains that are online.

Each update domain contains one-third of the total number of virtual machines in the availability set, rounded up to the nearest integer. In this case, there are seven virtual machines, so each update domain will contain three virtual machines, except for the last update domain which will contain only two virtual machines.

Therefore, the total number of virtual machines that are not in the offline update domain is:

19 update domains x 3 virtual machines per update domain + 2 virtual machines in the last update domain = 57 + 2 = 59

So, the answer is C. 6 App1 instances will remain available during a period of planned maintenance.