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MSTechie-7364 avatar image
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MSTechie-7364 asked singhh-msft edited

Autoshown feature along with Availability set for Azure VM

As per my knowledge Availability set option for VMs is for High Availability.

I want to have 2 VMs in an availability set. From what i observed in Azure portal , is when we create 2 VMs and attach to the same availability set , then i can shutdown both the VMs. I thought in Availability set, if one machine goes down , other machine is always up and running..like at any time ,atleast 1 VM is running.

Please correct my understanding here.

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learn2skills answered learn2skills edited

Hi @MSTechie-7364
Thank You for posting in Q & A.
Yes, you can shutdown both the VMs in availability set and if one machine is shutdown other machine is always up and running.

An availability set is a logical grouping of VMs that allows Azure to understand how your application is built to provide for redundancy and availability. We recommended that two or more VMs are created within an availability set to provide for a highly available application and to meet the 99.95% Azure SLA

There is no cost for the Availability Set itself, you only pay for each VM instance that you create.

Note- Behind the scenes an availability set is a kind of clustering with not sync between VMs, it just try to keep the VMs up and running.

Refer below screenshot.

78550-capture2.jpg


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singhh-msft answered singhh-msft edited

@MSTechie-7364 , thank you for reaching out to us. Happy to help. As I understand, you are referring to below situation:


78702-89.png


An availability set is a logical grouping of VMs that allows Azure to understand how your application is built to provide for redundancy and availability. It is basically your way to get an SLA of 99.95%, provided you put 2 or more VMs in it.:


78662-88.png


If you are interested in detailed working of Availability Sets, I would recommend you to check out How do availability sets work? and Understanding Azure Availability Sets.

Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform. Microsoft periodically updates the underlying Azure fabric that’s used to host VMs to patch security vulnerabilities and improve reliability and performance. These updates, which Microsoft refers to as planned maintenance events, are often performed without any impact to guest VMs. Sometimes, however, guest VMs must be rebooted to complete an update. To reduce the impact on guest VMs, the Azure fabric is divided into Update Domains to ensure that not all guest VMs are rebooted at the same time.

Unplanned maintenance events are those which involve a hardware or physical failure in the fabric, such as a disk, power, or network card outage. Azure automatically fails over guest VMs to a working physical host in a different Fault Domain when an error condition is detected, again aimed at ensuring availability.

So, if you are manually shutting down your VMs, they will (like you can still have 1 VM in availability set and which will not provide you the desired SLA as mentioned in here). In summary, this behavior is expected.


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Hi,

I had created an Availability Set and added 2 Azure VMs to that Availability Set. Now i shutdown both the servers and i see that none of the servers are coming back up ... since as per my understanding Availability set means , if 1 server goes down , atleast another server should be up and running ... and none of the servers were started back automatically .

Please correct my understanding

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@MSTechie-7364 , Availability Set takes care of "always up" only in planned and unplanned maintenance. Manual shutdown does not come under both of these types of maintenances. So, it is expected behavior.

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Hi @HarshitaSingh-MSFT
Why in my case, is the VM not getting started back up , even though they are part of availability.

So should i configure something , so that when the 2 VMs in one availability set goes down, atleast one VM comes back up

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@MSTechie-7364 , just checking in to see if I answered your question. If yes, pls accept it as an answer to help other community members.

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@MSTechie-7364 , just checking in to see if you got a chance to check my response.

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