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.
VxRail 4.5.070 introduced vSAN 6.6 which includes local site protection and site affinity for stretched clusters allowing unbalanced configurations. 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 that is host to the witness component which coordinates data placement between the preferred and secondary site. It also helps with the failover process. This is the third fault domain.
The vSAN storage policies that impact the VxRail Cluster configuration are:
- Primary Failures to Tolerate (PFTT)1/Failures to Tolerate (FTT) – for stretched clusters this rule has two possible values: 0 ensures protection on a single site; 1 enables protection across sites.
- Secondary Failures to Tolerate (SFTT) – This term is used in vSAN 6.6/VxRail 4.5.070 or later. The rule that defines the number of host and device failures that a virtual machine object can tolerate in the local site. Possible values: 0,1,2,3.
- Failure Tolerance Method- either RAID-1 (mirroring) used when performance is important or starting with vSAN 6.6/VxRail 4.5.070, RAID-5/6 (erase coding) used when capacity is important. For stretched clusters, this policy only applies to the SFTT setting. This failure tolerance method is the local file protection mode.
- Affinity (only applicable starting with vSAN 6.6/VxRail 4.5.070) - this policy is applicable when PFTT is set to 0. It is set to preferred or secondary to determine which site stores the vSAN object.