Seven hosts per availability zone is required for this design. In a vSAN ESA stretched cluster, data components are distributed across two data sites, while a witness site provides quorum. The first requirement is a secondary level of resilience of FTT=2 using RAID-6 erasure coding. RAID-6 requires enough fault domains to place data and parity components while tolerating two failures. The second requirement is more restrictive: vSAN must be able to regain its prescribed level of resilience after a sustained host outage. This means the site must have enough remaining placement capacity to rebuild components and return objects to policy compliance after a host is lost. A six-host site can satisfy the RAID-6 placement minimum, but it does not provide the additional host needed to rebuild cleanly after a sustained host outage. Therefore, the design requires seven hosts in each availability zone. This produces fourteen data hosts across the stretched cluster, plus the witness. Reference topics: vSAN ESA Stretched Cluster Design, Secondary Level of Resilience, RAID-6 Erasure Coding, Availability Zone Host Count.

The minimum total number of nested fault domains is six. In a two-node vSAN design, site disaster tolerance is provided through host mirroring between the two data hosts, with a witness maintaining quorum. The requirement adds another layer: the solution must also survive a disk group or local storage failure in addition to a host failure. vSAN's two-node host mirroring rule states that each data host must have at least three disk groups, or three disks in a storage pool, to use the rule. In ESA, the equivalent construct is disks in the storage pool rather than OSA-style disk groups. Because there are two data hosts, and each data host requires three nested placement units, the minimum total is 3 + 3 = 6 nested fault domains. Four would not meet the host-mirroring rule requirement, and two would only represent the two hosts without local storage fault isolation. Reference topics: vSAN Two-Node Cluster, Host Mirroring, Nested Fault Domains, vSAN ESA Storage Pool Placement.

The prerequisite is to enable vSAN on the non-vSAN cluster so it can operate as a vSAN compute cluster. A vSAN compute cluster is a vSphere cluster that does not contribute local vSAN capacity, but it has vSAN functionality enabled so it can mount a remote vSAN datastore provided by a vSAN Storage Cluster. The Datastore Management option under the vSAN section appears only when the consuming cluster is prepared for vSAN datastore sharing. Without vSAN enabled in compute-cluster mode, the vSphere cluster remains a conventional VMFS-backed cluster and cannot use the vSAN remote datastore management workflow. The cluster does not need to be in a different VCF Workload Domain, and there is no requirement that the storage cluster maintain exactly 80% free space before the option appears. A fourth host is also not required simply to mount remote vSAN capacity. The key requirement is enabling vSAN capability on the client cluster so it can communicate with and consume the remote ESA datastore. Reference topics: vSAN Compute Cluster, Remote vSAN Datastore, Datastore Management, vSAN Storage Cluster.

Explanation:
Full Clone from Volume Snapshot
4kn Support
Online Volume Expansion
Offline Volume Expansion
Static Persistent Volume
VMware Kubernetes Service persistent volumes use vSphere storage integrations such as CNS and pvCSI to provide Kubernetes storage services backed by vSphere datastores and storage policies. The supported persistent volume capabilities include Static Persistent Volume, which allows pre-existing storage to be consumed by Kubernetes workloads; Offline Volume Expansion and Online Volume Expansion, which allow persistent volumes to be resized depending on workload and filesystem state; Full Clone from Volume Snapshot, which enables a new volume to be created from an existing snapshot; and 4kn Support, which supports modern 4K native storage devices where the underlying stack supports them. Storage vMotion with attached Persistent Volumes is not supported because moving an attached Kubernetes persistent volume while it is in active use can break attachment and consistency assumptions. Storage DRS is also not a supported persistent volume capability in this VKS context. Flash Read Cache is not part of the supported VKS persistent-volume capability set. Reference topics: VMware Kubernetes Service Storage, CNS, pvCSI, Persistent Volume Capabilities, Volume Expansion, Volume Snapshot Clone.

A witness host for a 2-node vSAN cluster must be external to the 2-node vSAN-enabled data cluster.
The witness provides quorum and must be placed outside the protected data-node pair so that it can arbitrate availability during host or site failure. If the proposed witness host resides inside the 2-node vSAN enabled cluster, it is not eligible and therefore does not appear as an available selection during the replacement workflow. Having a VMkernel adapter configured with vSAN traffic enabled is required for witness communication, so that is not the cause of the issue. Connectivity to the data hosts is also expected and does not make the witness invalid. A witness host already assigned to another 2- node vSAN cluster is not necessarily disqualified because shared witness configurations are supported within scale and sizing limits. The core eligibility issue is that the witness host cannot be a member of the same 2-node vSAN cluster that it is intended to witness. Reference topics: 2-Node vSAN Cluster, Witness Host Placement, Replace Witness Host, Shared Witness Support.