This planning guide provides best practices and requirements for using stretched cluster with a VxRail Appliance. This guide assumes that the reader is familiar with the vSAN Stretched Cluster Guide. This guide is for use with a VxRail Appliance only.

The vSAN stretched cluster feature creates a stretched cluster between two geographically separate sites, synchronously replication data between sites. This feature allows for an entire site failure to be tolerated. It extends the concept of fault domains to data center awareness domains.

The following is a list of the terms that are used for vSAN stretched clusters:

• Preferred or Primary site – one of the two data sites that is configured as a vSAN fault domain.
• Secondary site – one of the two data sites that is configured as a vSAN fault domain.
• Witness host – a dedicated ESXi host or vSAN witness appliance can be used as the witness host. The witness components are stored on the witness host and provide a quorum to prevent a split-brain scenario if the network is lost between the data sites. This is the third fault domain.

The vSAN storage policies that impact stretched cluster are:

• Dual site mirroring (stretched cluster) – enables protection across sites.
• None – keep data on Preferred (stretched cluster) – Keep data on primary site only, no cross-site protection.
• None – keep data on Non-preferred (stretched cluster) – Keep data on secondary site only, no cross-site protection.
• Failures to Tolerate – defines how many disk or node failures can be tolerated for each site. For stretched cluster, it’s ‘2n+1’ (n is the number to tolerate). For erasure coding, it’s 4 or 6 (1 or 2 failures respectively). All-Flash is required for erasure coding (RAID5 or 6).