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.

Note: Since any storage media can be subject to catastrophic failure requiring complete replacement, a VSA environment’s virtual machines and user data should always be periodically backed up to media external to the VSA environment. The definition of acceptable frequency of such backups is outside the scope of this document and is left to customer management discretion. For more information on this topic, please consult the VMware Knowledge base or contact VMware Technical Support.

Centralized or Distributed Storage

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.

Figure 1. Centralized Storage Arrays in Comparison with Distributed Shared Storage
Monolithic arrays vs. distributed shared storage

Local Storage or Networked Storage

In a typical storage configuration, your host uses local storage disks, or has access to networked storage. A VSA cluster utilizes the hard disks that are local to each ESXi host.

Local storage
Local storage can be internal hard disks located inside your ESXi host, or it can be external storage systems located outside and connected to the host directly through protocols such as SAS or SATA. Typically, local storage does not require a storage network to communicate with your host.

Local storage devices do not support sharing across multiple hosts. A datastore on a local storage device can be accessed by only one 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.

Networked storage devices are shared. Datastores on networked storage devices can be accessed by multiple hosts concurrently.