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Set up a vSAN 2-node cluster for small or remote deployments

Use a vSAN 2-node cluster for small and branch office deployments. Use a cluster with two physical hosts for data storage and a third server as a witness appliance.

VMware's vSAN has forged a place in modern data centers as a mature storage service, but small and branch office...

deployments can also take advantage of this technology with a vSAN 2-node cluster setup that uses two physical hosts and a witness appliance.

VMware introduced vSAN in vSphere 5.5, and it has since become a capable storage technology for vSphere customers. Organizations primarily use it to replace or add to enterprise storage deployments.

If you're the administrator of a smaller deployment or remote office/branch office (ROBO) deployment, however, you can use vSAN with a special vSAN 2-node cluster setup. The minimum for a vSAN cluster is three nodes. You can also set up a cluster with two physical hosts to store your data and a third server as a witness appliance.

Establish vSAN cluster hosts

Three hosts is the absolute minimum for a normal cluster. You can store copies of your data on two of these hosts and store a witness object on the third host. Figure A shows two ESXi hosts and one virtual disk stored on those hosts.

This is the default storage policy in vSAN. It will tolerate one host failure without causing any problems, better known as primary failures to tolerate (PFTT) with PFTT = 1. If you lose one of the hosts or disks in a host, you'll always have access to two out of the three components, which form more than 50% of the object.

ESXi hosts and virtual disk witness
Figure A. Three ESXi hosts and one virtual disk witness form a minimal cluster.

Although these three hosts form the absolute minimum, it's not necessarily the most practical choice. When you want to place a host in maintenance, there is nowhere to move your data. If you have four hosts, you can move the data to a fourth host to retain high availability during maintenance. If one node fails for more than one hour, vSAN will rebuild the data to a high availability level of PFTT = 1 automatically. This will keep you protected while you repair or replace the host.

A vSAN 2-node cluster configuration with a virtual witness appliance might seem worse for availability due to the number of hosts, but you can place a host in maintenance and retain the availability level. In most cases, this setup can replace a single host setup.

The vSAN 2-node cluster setup enables you to run two hosts that store data and run VMs. This provides redundancy for the data, and running your VMs on both hosts provides high availability. This is a significant improvement compared to a single host setup.

Licensing options lend flexibility

You can set up a vSAN 2-node cluster with a ROBO license, as well as a regular vSAN license, vSphere Essentials Kit or vSphere Essentials Plus Kit.

VMware prices ROBO licenses per VM and sells them in packages of 25 licenses. You can share a 25-pack of licenses across multiple locations; for example, five remote offices each running five VMs.

Each remote office is limited to a maximum of 25 VMs under the ROBO licensing model. If you run more than 25 VMs at a remote office, you'll need vSAN Standard, Advanced or Enterprise licensing. You can license any number of hosts with vSAN for ROBO Standard, Advanced or Enterprise as long as the number of VMs running on a vSAN cluster at a single location is 25 or fewer.

Licensing flexibility is part of what makes a vSAN 2-node cluster deployment affordable. Additionally, the hardware requirements are different than those for a larger cluster. You'll still need two servers, and each server needs flash-based storage for the caching tier and a flash device or magnetic disk for the capacity tier.

You can set up the required 10 Gigabits Ethernet (GbE) network with a crossover cable between the hosts, which vSphere has supported since version 6.5. For a hybrid service with a flash cache and a magnetic disk, less than 10 GbE could work, but having 10 GbE is better. For all-flash deployments, 10 GbE networking is required.

In Figure B, a 10 GbE network adapter connects two hosts with a crossover cable that eliminates the need to purchase a 10 GbE switch. It limits the number of hosts to two, but that was the goal. The hosts must be able to contact each other, as well as vCenter for management and the witness host, so you can use the already existing network for that purpose.

Diagram of virtual disk connections
Figure B. A 10 GbE network adapter crossover connects two hosts.

Set up a vSAN 2-node cluster

The setup for a vSAN 2-node cluster deployment requires a third node to store witness files. This doesn't have to be a physical server. VMware has a virtual appliance that you can deploy in an already available hypervisor.

Figure C shows a common scenario in which two 2-node clusters are deployed in a branch office. You can then place a witness appliance on a hypervisor at headquarters.

Diagram of virtual disk copies and virtual disk witnesses
Figure C. Place a witness appliance on a hypervisor

This scenario with the witness appliance at headquarters isn't the only possible method. You can also run the witness appliance in a cloud deployment with a cloud provider.

This method typically uses VMware vCloud Air, but you could also use another cloud provider that lets you run a virtual ESXi host in their cloud. As long as the two nodes of your cluster can reach the witness node via a layer three network connection, it doesn't matter where the witness appliance actually runs.

Use the vSphere Client

Use vSphere Client to set up the vSAN cluster object. In the client, find the option for Fault Domains and Stretched Clusters. Technically, this cluster is another form of a stretched cluster with a witness node, but an actual stretched cluster with more than two nodes would require an Enterprise vSAN license.

In Figure D, you can see the setup of a vSAN 2-node cluster with one host in the primary fault domain, and the other host in the secondary fault domain. These point to the witness appliance that you deployed in a third ESXi host.

VSphere Client
Figure D. Select Fault Domains and Stretched Cluster.

It's always better to have at least two disk groups per server rather than one disk group in smaller clusters. A cache device and one or more capacity devices form a disk group. If your server loses access to the disk group, there is nowhere else to recreate the missing components. With an extra disk group in your server and enough disk space, vSAN can recreate the absent objects in the available disk group.

In a much larger cluster with 12 nodes, for example, it's almost always possible to rebuild the data from a lost disk group in another host. With just two nodes, it isn't possible to rebuild the data on the other host if you lose the single disk group in that host. If you have a second disk group and there is space available, then vSAN will rebuild the data there.

VSAN is a powerful storage service, but with a vSAN 2-node cluster setup, even small and ROBO deployments can take advantage of it.

This was last published in March 2018

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