The Fault Domains functionality is yet another crucial aspect of vSAN. If your vSAN cluster is spread over several racks or blade server chassis, fault domains allow you to safeguard against rack or chassis failure.
One or more vSAN hosts are grouped together into fault domains based on where they are physically located in the data center. When fault domains are configured, vSAN is able to withstand failures of not just individual hosts, capacity devices, network links, or network switches but also failures of entire physical racks.
The Failures to tolerate policy for the cluster depends on the number of failures a virtual machine is provisioned to tolerate. vSAN can tolerate a single failure of any kind and of any component in a fault domain, including the failure of a complete rack, when a virtual machine is configured with the Failures to tolerate setting to 1 (FTT=1).
When fault domains are set up on a rack and a new virtual machine is provisioned, vSAN makes sure that protection objects like replicas and witnesses are put in various fault domains. For instance, if the Failures to Tolerate setting on a virtual machine's storage policy is set to N (FTT=n), vSAN requires a minimum of 2*n+1 fault domains in the cluster. This policy stores copies of the relevant virtual machine objects across many racks when virtual machines are provisioned in a cluster with failure domains.
To support FTT=1, a minimum of three fault domains are needed. Configure the cluster with four fault domains or more for the best results. A cluster with three fault domains has the same restrictions that a three host cluster has, such as the inability to re-protect data after a failure and the inability to use the Full data migration mode.
When a rack fails, all resources including the CPU, memory in the rack become unavailable to the cluster. To reduce the impact of a potential rack failure, configure fault domains of smaller sizes. Increasing the number of fault domains increases the total amount of resource availability in the cluster after a rack failure.
Use these best practices when working with fault domains -
- When designing a fault domain, place a consistent number of hosts in each fault domain.
- A fault domain can have any number of hosts. There must be at least one host in each fault domain.
- When moved to another cluster, vSAN hosts retain their fault domain assignments.
- Set up the vSAN cluster with a minimum of three fault domains. Configure four or more fault domains for the best results.
- A host is considered as residing in its own single-host fault domain if it is not a part of any fault domain.
- Every vSAN host does not have to be assigned to a failure domain. Consider making equal-sized fault domains if you choose to employ them to safeguard the vSAN environment.
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