Clarification on vSAN "Fault domain failures to tolerate” as displayed under Cluster - Configure - Fault Domains

Products

• VMware vSAN

Issue/Introduction

Under vSAN Cluster - Configure - Fault Domains, the value for "Fault domain failures to tolerate” is not configurable and reflects only a maximum cluster FTT (host failures to tolerate), only the storage policy applied to individual objects or VMs dictates the actual FTT.

 

For single site clusters, the following table shows all available storage policy FTT options and the minimum hosts required:

RAID Configuration	Failures to Tolerate (FTT)	Minimum Hosts Required
RAID-1 (Mirroring)	1	2
RAID-5 (Erasure Coding)	1	4
RAID-1 (Mirroring)	2	5
RAID-6 (Erasure Coding)	2	6
RAID-1 (Mirroring)	3	7

 

As an example, a RAID5/FTT1 storage policy requires 4 fault domains (which are individual hosts if configuration type is single site) to be fully healthy and compliant, and can tolerate only one fault domain loss (FTT1).  Therefore, a 4 node single site cluster will always reflect "Fault domain failures to tolerate” of 1.

 

Any single site cluster larger than 7 nodes can tolerate at least 3 fault domain losses, the maximum FTT storage policy that can be applied to any single site cluster is RAID1/FTT3.  Therefore, a 7+ node single site cluster will always reflect "Fault domain failures to tolerate” of 3.

 

In a stretched cluster we have a primary FTT (PFTT) consisting of each site and a secondary FTT (SFTT) configured per object when using a site mirroring storage policy, where each site mirror has its own fault tolerance (RAID1, RAID5, or RAID6).  The "Fault domain failures to tolerate" in a stretched cluster reflects the maximum SFTT, so a 8 node stretched cluster (4 nodes at each site) will reflect 1 and a 14+ node stretched cluster (7 nodes at each site) will reflect 3.

Environment

6.0

7.0

8.0

9.0

Resolution

Reference the following documentation on all available storage policy options:

What are vSAN Policies