The vSAN 2 Node configuration was introduced in vSAN 6.1. A vSAN 2 Node Cluster is a specific configuration implemented in environments where a minimal configuration is a key requirement.
Two-node vSAN clusters are often used for remote office/branch office environments, typically running a small number of workloads that require high availability. A two-node vSAN cluster consists of two hosts at the same location, connected to the same network switch or directly connected. A third host acts as a witness host, which can be located remotely from the branch office. Usually the witness host resides at the main site, with the vCenter Server. A single witness host can support up to 64 two-node clusters. The number of clusters supported by a shared witness host is based on the host memory.
In the diagram below, a simple vSAN 2 node configuration is shown.
Each node is configured as a vSAN Fault Domain. The supported configuration is 1+1+1 (2 nodes + vSAN Witness Host). With two Fault Domains available, virtual machines deployed on vSAN 2 Node Clusters typically have mirrored data protection, with one copy of data on Node 1, a second copy of data on Node 2, and the Witness component placed on the vSAN Witness Host or on a vSAN Witness Appliance.
In the event of a node or device failure, a full copy of the virtual machine data is still available on the alternate node. Because the alternate replica and Witness component are still available, the virtual machine remains accessible on the vSAN datastore. In cases where the virtual machine was running on the failed node, vSphere HA can handle this task if properly configured.
Each 2-Node deployment until vSAN 7 required a dedicated witness appliance. vSAN 7 Update 1 introduces the capability to use a shared witness appliance instance across multiple 2-Node deployments. Up to 64 2-node clusters can share a single witness appliance. This enhancement further simplifies design, eases manageability, and operations.
Configurations for the VMware vSAN 2 Node Cluster require vSphere 6.0 Update 1 (U1) or later. This needs ESXi 6.0 U1 and vCenter Server 6.0 U1 and vSAN version 6.1.
VMware vSAN 2 Node Clusters are supported on both Hybrid configurations and All-Flash configurations.
vSAN Witness Host -
vSAN Witness Host can be configured as either physical ESXi host or vSAN Witness Appliances. The vSAN Witness Appliance must run on an ESXi 5.5 or higher host. If the vSAN Witness Appliance is used for a vSAN 6.7 2 Node cluster, the ESXi 5.5 or higher host must also meet the CPU requirements of vSphere 6.7.
As of vSAN 7 U1, the witness node should be upgraded first (to maintain backward compatibility). Upgrading of the witness can be accomplished through VUM but is not supported by vLCM at this time. All vSAN clusters participating in a shared witness must use the new on-disk format (ODF) version associated with vSAN 7 U1 (v13).
Limitations on vSAN 2 node cluster -
- In a vSAN 2 Node Clusters, there are only 3 Fault Domains. Two fault domains are configured (typically referred to as the Preferred and Secondary) and the Witness fault domain is implied. Standard vSAN configurations can be comprised of up to 32 Fault Domains.
- Stretched Clusters and 2-node configurations are not supported in combination with HCI Mesh.
- Backup and restore, clones, snapshots, storage vMotion, and replication of a vSAN Witness Host are not supported.
That's it for now. In the next few articles we will discuss about design considerations of vSAN 2 node cluster and the steps we need to follow to configure it.
I hope you find this article informative.
Thank you for reading!
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