vSAN has very useful feature to provide availability which is called Fault Domain. vSAN Fault Domain works on the rack awareness & provides rack level availability for your vSAN Cluster Hosts. Fault domains allows you to protect against rack or chassis failure if your vSAN cluster spans across multiple racks or blade server chassis. You can create fault domains and add one or more hosts to each fault domain.
So What is Fault Domain & How Fault Domain Works in vSAN:
A fault domain consists of one or more vSAN hosts grouped according to their physical location in the data center. When configured, fault domains enable vSAN to tolerate failures of entire physical racks as well as failures of a single host, capacity device, network link, or a network switch dedicated to a fault domain.
The Primary level of failures to tolerate policy for the cluster depends on the number of failures a vm is provisioned to tolerate. When a vm is configured with the Primary level of failures to tolerate set to 1 (PFTT = 1), vSAN can tolerate a single failure of any kind and of any component in a fault domain, including the failure of an entire rack.
When you configure fault domains on a rack and provision a new virtual machine, vSAN ensures that protection objects, such as replicas and witnesses, are placed in different fault domains. A minimum of three fault domains are required to support PFTT=1. For best results, configure four or more fault domains in the cluster.
So now you know what is Fault Domain & How it works. Let’s see How to configure vSAN Fault Domain:
Select your vSAN Cluster.
Go to Configure Tab.
Expand vSAN Section.
Select Fault Domains.
Here you will see configured Fault Domain.
To create Fault Domain click on the Icon Create Fault Domain above Fault Domains Box as shown.
You will get Fault Domain wizard.
Provide Name for your Fault Domain. Choose name based on your physical server placements so you can easily identify and segregate hosts.
Select the ESXi host to add to the Fault Domain.
Click OK to save.
Once you click OK you will see that Fault Domain is created & your host is associated with Fault Domain.
So using this way you can easily create Fault Domain in vSAN & achieve availability on Physical Racks basis. It is recommended to configure fault domain so that if one of your rack fails you will still have data from another racks.
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Information source: VMware vSAN Documentation.