VMware Product Primer - vSphere Storage Appliance
Perhaps one of the most vexxing challenges for SMBs has been how to gain access to some of vSphere's more advanced features without having to run out and buy a SAN to use as a shared storage device. Shared storage has traditionally been a requirement for many of vSphere's workload migration and high availability mechanisms, although reliance on shared storage for some of these features has decreased a bit in vSphere 5.1.
VMware's vSphere Storage Appliance (VSA) is a tool that can transform existing, local server storage into shared storage that can be shared by up to three vSphere hosts to create a single cluster, thus enabling some high-end vSphere capabilitirs without having to invest in an expensive SAN. Administrators need only to repurpose local storage on the vSphere server to make the service work. Witj VSA, workload migration and availability needs can be much more easily addressed than without.
With VSA, local RAID controllers on each host continue to protect data from data loss, but the VSA's own networking mirroring services copy data between nodes, making it possible for services to remain available even if a single host fails.
VSA is included in vSphere Essentials plus and can also be purchased by itself. Up to 150 VSA clusters can be managed with an existing vCenter license.