Nutanix operates in the user space while Virtual SAN (which includes EVO:RAIL, of course) is embedded in the hypervisor kernel. Each design offers unique challenges and choices when designing, deploying and managing hyper-converged systems.
VSAN is directly plumbed into the compute cluster. The storage available for the virtual machines running on VSAN hosts is inherently limited to the local attached storage of the hosts that make up the vSphere compute cluster. For example, you can’t extend the VSAN container to other compute clusters, and you can only have one container per compute cluster.
Depending on vSphere and application licensing (e.g., databases), VSAN can potentially create silos of capacity and performance.
Nutanix, on the other hand, is not bound to a vSphere compute cluster. So while you could architect a solution similar to VSAN, you have the choice to create one large storage clusters – regardless of licensing of the compute layer.
To illustrate, let’s use the above example of 9 hosts, each with 2 TB of useable capacity per node for both VSAN and Nutanix. Also, we’ll architect for fault tolerance of 1.
With VSAN clusters you would have 3 separate containers of 4 TB, accounting for space for the node to fail and rebuild. Nutanix can aggregate of all the nodes, yielding 1 large container of 16 TB, also accounting for space for the node to fail and rebuild.
Normally you only need 3 nodes with either solution to get your cluster up and running. If you want the ability to lose more than one node at anyone time the minimum starting number of nodes in the cluster needs to be higher. Fault tolerance(FT) of 2 means your cluster can lose two nodes at anyone time and avoid an outage. It wouldn’t be possible to achieve FT=2 with the smaller VSAN clusters because you would need at least 5 nodes in one cluster to survive the loss of the two nodes. Nutanix also needs a minimum of 5 nodes for FT=2 but Nutanix would have access to all 9 nodes across the multiple vSphere compute clusters so it would be possible to configure this.
Also, having a larger cluster with Nutanix means you can more easily share the most expensive resource, flash. If flash is available, all 9 nodes will have access to the flash.