The software defined datacenter offerings from VMware are continuing to mature and offer exciting new features and functionality. There have been leaps and bounds advancements in VMware’s software defined storage and networking. In the world of software defined storage, VMware’s vSAN technology is certainly taking the world by storm with ever increasing deployment numbers as well as new features with each release. One of the features announced with vSAN 6.1 was the stretched cluster and the 2-node vSAN cluster. Both of these vSAN functionalities are made possible by the vSAN Witness Appliance. What is the vSAN Witness Appliance and how is it deployed?

What is the VMware vSAN Witness Appliance?

In VMware vSAN architecture, the “witness” component is used to provide an availability mechanism to virtual machines. The witness component arbitrates ownership of the virtual machine resources in the event of a failure. If greater than 50% of the components that make up a virtual machine’s storage object are available, the virtual machine is still accessible. If less than 50% of the virtual machine components are available, then it will no longer be accessible to the vSAN cluster/datastore. So, “witness” objects play a crucial role in ensuring the “greater than 50%” rule is in effect for vSAN component objects and determining virtual machine object ownership in a failure event.

Having the ability to deploy the VMware vSAN Witness Appliance holds several advantages. From a licensing perspective, the VMware vSAN appliance is advantageous as the license is “built into” the vSAN appliance. VMware has provided this license in the appliance free of charge. This keeps customers from having to dedicate a physical ESXi host as a witness host and the licensing that would be required for the dedicated ESXi host. Additionally, it brings the new capabilities of the 2-node vSAN cluster and the stretched vSAN cluster.

Taking a 2-node vSAN cluster as an example, the two vSAN cluster nodes house the two replica copies of the actual VMDK itself, the vSAN Witness Appliance houses the witness disk which is simply metadata that consumes 2MB of disk space on the appliance. For this reason, the vSAN Witness Appliance requires much less bandwidth and performance than the vSAN ESXi hosts.

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The VMware vSAN appliance is an interesting appliance in itself as it is a “nested ESXi” server running in appliance form from VMware. This is the first nested ESXi solution that is supported from VMware and comes preconfigured with the disk configuration and network adapters to connect to the VMware vSAN infrastructure.

Deploying the VMware vSAN Witness Appliance

How is the VMware vSAN Witness Appliance deployed? It comes preconfigured from VMware as an OVA appliance that is easily deployed in a few minutes. Login to your VMware portal and download the appliance to be deployed.


Download the VMware vSAN appliance

Once you have the appliance downloaded, it can be deployed via the standard OVF/OVA deployment process in vCenter. Browse to the downloaded VMware vSAN appliance OVA file.


Choose the downloaded VMware OVA file to deploy the vSAN Witness Appliance

On the Select name and location screen, we choose the name of the vSAN Witness Appliance, the target Datacenter and folder location in which to deploy


Choose the Name Datacenter and folder

On the Select a resource screen, you choose the VMware compute cluster that will house the vSAN Witness Appliance.


Choose the VMware compute cluster

In mid deployment, we review the general configuration set for the OVA appliance itself before proceeding.


Review VMware vSAN Witness Appliance template details

Next, accept the EULA for the vSAN Witness Appliance.


Accept the EULA

On the select configuration screen, you choose the configuration sizing for the vSAN Witness node. The sizing configuration is broken down as follows:

  • Tiny (10 VMs and fewer) – 2 vCPUs, 8GB RAM, 8GB ESXi boot volume, one 10GB SSD, and one 15GB HDD
    • Maximum of 750 witness components
  • Medium (up to 500 VMs) – 2 vCPUs, 16GB RAM, 8GB ESXi boot volume, one 10GB SSD, and one 350GB HDD
    • Maximum of 21,000 witness components
  • Large (more than 500 VMs) – 2 vCPUs, 32GB RAM, 8 GB boot volume, one 10GB SSD, three 350GB HDDs
    • Maximum 45,000 witness components


Choose the deployment sizing – Available in Tiny, Medium, and Large depending on the number of VMs

On the select storage screen, we


Choose the disk provisioning and target datastore for the vSAN Witness Appliance


Choose the network configuration for the Witness and Management networks and the portgroups they are attached to


Configure the root password for the vSAN Witness Appliance


Review the configuration and begin deploying the appliance

Adding the vSAN Witness Appliance to vCenter Server

After the vSAN Witness Appliance is deployed it can be added to vCenter Server. Interestingly after adding the vSAN Witness Appliance to vCenter, the ESXi host will appear as a light shade of blue as compared to the normal grey color of ESXi workload hosts.


Notice the different color of the vSAN Witness Appliance ESXi host

Once the appliance is up and running, it can be used in configuring a stretched or a 2-node vSAN cluster.


Configuring a 2-node vSAN cluster by using the new vSAN Witness Appliance

Concluding Thoughts

VMware vSAN has certainly matured and making great strides in the HCI space. The new offerings for stretched vSAN clusters as well as 2-node vSAN clusters drastically increases the options that organizations have in configuring vSAN. The key to those technologies lies in the functionality of the vSAN Witness Appliance. It is a very unique component to VMware vSAN infrastructure. It provides the needed witness components that provide the quorum votes to prevent “split brain” and other scenarios. The free licensing provided from VMware adds to the potential cost savings benefits as well.

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What is VMware vSAN Witness Appliance and Deployment
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