Unable to upgrade vSAN 2 node stretch cluster from 7.0 to 8.0 without witness
Article ID: 394784
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Issue/Introduction
Upgrading a vSAN 2 node stretch cluster fails without the witness node in the cluster
"There is currently 1 usable fault domain. The operation requires 2 more usable fault domains."
Cause
Stretched clusters use fault domains to provide redundancy and failure protection across sites. Each site in a stretched cluster resides in a separate fault domain.
A stretched cluster requires three fault domains: the preferred site, the secondary site, and a witness host. Each fault domain represents a separate site. When the witness host fails, is absent, or enters maintenance mode, vSAN considers it a site failure.
The witness host serves as a tiebreaker when a decision must be made regarding availability of datastore components when the network connection between the two sites is lost
Resolution
When upgrading a stretch cluster using a shared witness, always upgrade the witness after all the ESXi hosts in the cluster.
When upgrading a stretch cluster using a standalone witness with target builds of 7.0 or higher always upgrade the witness host before the ESXi hosts.
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