Design the configuration of hosts and management nodes for best availability and tolerance to consumption growth.

Sizing the vSAN Cluster for Failures to Tolerate

You configure the Primary level of failures to tolerate (PFTT) attribute in the VM storage policies to handle host failures. The number of hosts required for the cluster is calculated as follows: 2 * PFTT + 1. The more failures the cluster is configured to tolerate, the more capacity hosts are required.

If the cluster hosts are connected in rack servers, you can organize the hosts into fault domains to improve failure management. See Designing and Sizing vSAN Fault Domains.

Limitations of a Two-Host or Three-Host Cluster Configuration

In a three-host configuration, you can tolerate only one host failure by setting the number of failures to tolerate to 1. vSAN saves each of the two required replicas of virtual machine data on separate hosts. The witness object is on a third host. Because of the small number of hosts in the cluster, the following limitations exist:

  • When a host fails, vSAN cannot rebuild data on another host to protect against another failure.

  • If a host must enter maintenance mode, vSAN cannot reprotect evacuated data. Data is exposed to a potential failure while the host is in maintenance mode.

    You can use only the Ensure data accessibility data evacuation option. The Evacuate all data option is not available because the cluster does not have a spare host that it can use for evacuating data.

    As a result, virtual machines are at risk because they become inaccessible if another failure occurs.

Balanced and Unbalanced Cluster Configuration

vSAN works best on hosts with uniform configurations.

Using hosts with different configurations has the following disadvantages in a vSAN cluster:

  • Reduced predictability of storage performance because vSAN does not store the same number of components on each host.

  • Different maintenance procedures.

  • Reduced performance on hosts in the cluster that have smaller or different types of cache devices.

Deploying vCenter Server on vSAN

If you deploy vCenter Server on the vSAN datastore, you might not be able to use vCenter Server for troubleshooting, if a problem occurs in the vSAN cluster.