A VSA cluster is a virtual alternative to expensive SAN systems. While SAN systems provide centralized arrays of storage over a high-speed network, a VSA cluster provides a distributed array that runs across several physical servers and utilizes local storage that is attached to each ESXi host.

SAN systems provide centralized arrays of storage that are managed by several storage processors. The vSphere Storage Appliance provides a distributed approach to storage, where the storage array is dispersed over several ESXi hosts and is accessible over the network.

Centralized Storage Arrays in Comparison with Distributed Shared Storage
Monolithic arrays vs. distributed shared storage

Servers can access storage through local hard disks or through hard disks that are attached to NAS or SAN systems. A VSA cluster utilizes the hard disks that are local to each ESXi host.

Local storage

Local storage consists of internal hard disks located inside your ESXi host.

Networked storage

Networked storage consists of external storage systems that your ESXi host uses to store virtual machine files remotely. Typically, the host accesses these systems over a high-speed storage network.