What is PFTT and SFTT in VMware vSAN 6.6

In vSAN 6.6, new storage policy features have been added for stretched clusters. These features provide failure protection for stretched clusters: 
Local Protection for Stretched Clusters

• Primary level of failures to tolerate (PFTT) replaces the previous Failures to Tolerate (FTT). It applies to tolerate the failures across sites of streched cluster

• Secondary level of failures to tolerate (SFTT) is a new rule that applies to tolerate the failures within each site of a stretched cluster. 

Note: SFTT is only available when streched cluster is configured, as vSAN capability for creating the vSAN Storage Policies

• The failure tolerance method is tied to the SFTT rule as it defines how failure tolerance is achieved within each site but not across sites.
• One can have PFTT = 0, SFTT = 1 or 2 either they can have PFTT = 1, SFTT = 1 or 2.

RAID 5/6 Erasure Coding in Stretched Clusters

• RAID 5/6 erasure coding is only supported within each site.
• RAID 1 mirroring is supported cross-site.

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