“Fault domain” is a term that comes up often in availability discussions. In IT, a fault
domain usually refers to a group of servers, storage, and/or networking components
that would be impacted collectively by an outage. A common example of this is a server
rack. If a top-of-rack switch or the power distribution unit for a server rack would fail, it
would take all the servers in that rack offline even though the server hardware is
functioning properly. That server rack is considered a fault domain.
Each host in a vSAN cluster is an implicit fault domain. vSAN automatically distributes
components of a vSAN object across fault domains in a cluster based on the Number of
Failures to Tolerate rule in the assigned storage policy.
When determining how many hosts or Fault Domains a cluster is comprised of, it is important to remember the following:
• For vSAN objects that will be protected with Mirroring, there must be 2n+1 hosts or Fault Domains for the level of protection chosen.
- Protecting from 1 Failure would require (2×1+1) or 3 hosts.
- Protecting from 2 Failures would require (2×2+1) or 5 hosts.
- Protecting from 3 Failures would require (2×3+1) or 7 hosts.
• For vSAN objects that will be protected with Erasure Coding, there must be 2n+2 hosts or Fault Domains for the level of protection chosen.
- RAID5 (3+1) requires (2×1+2) or 4 hosts.
- RAID6 (4+2) requires (2×2+2) or 6 hosts.
Also consider that the loss of a Fault Domain, or hosts when Fault Domains are not configured, could result in no location to immediately rebuild to. VMware recommends having an additional host or Fault Domain to provide for the ability to rebuild in the event of a failure.
Also consider that the loss of a Fault Domain, or hosts when Fault Domains are not
configured, could result in no location to immediately rebuild to. VMware recommends
having an additional host or Fault Domain to provide for the ability to rebuild in the
event of a failure.
To mitigate this risk, we can place the servers in a vSAN cluster across server racks and
configure a fault domain for each rack in the vCenter\vSAN UI. This instructs vSAN to
distribute components across server racks to eliminate the risk of a rack failure taking
multiple objects offline. This feature is commonly referred to as “Rack Awareness”. The
screenshot shows component placement when three servers in each rack are configured
as separate vSAN fault domains.
Create Fault Domains
Victor Wu
Chief Architect, Blogger, Author at Dell EMC Knowledge Sharing & Packt
Victor Virtualization 
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