If I want to configure 3 failures tolerated, do I need to have 2*3+1=7 hosts in the vSan cluster? If so - can anyone explain why, exactly?
VSAN is a "RAIN" architecture, which is a redundant array of nodes (hosts). Assuming the failure of an entire node (host), you would need 2n+1 nodes to protect against entire server failure. In the case of tolerating 3 failures, that would equate to requiring 7 nodes. Of these 7 nodes:
- 4 nodes contain copies of the data
- 3 nodes are witnesses
If you were to lose 3 nodes that contained data, you would still have one node with data remaining.
You can also read my post on VSAN here: http://wahlnetwork.com/2013/08/26/vsphere-5-5-improvements-part-4-virtual-san-vsan/
Also, Duncan Epping has a GREAT post here: http://www.yellow-bricks.com/2013/10/24/4-minimum-number-hosts-vsan-ask/