Storage virtualization was initially introduced via the volume manager, which allowed a user to slice and dice a hard drive into multiple volumes. Over time it evolved to include functions like RAID, thin provisioning and snapshots. Marks says most operating systems include volume managers that can create RAID sets and divide them into volumes.
The most common platform for external storage virtualization is the dedicated storage appliance, which sits in the SAN data path between host servers and storage arrays. Because data access passes through the appliance, most organizations will deploy appliances in redundant pairs or clusters so the failure of one appliance won’t cut off access to the storage system.
Storage vendors have developed software called virtual storage appliances (VSAs). These perform some of the same functions as physical appliances but are instead delivered as a virtual machine that runs on a hypervisor. Because many of the most interesting hypervisor features, such as live migration and high availability, require some sort of shared storage, VSAs make these features available to branch offices and SMB server rooms that can’t justify the cost and complexity of traditional SANs.
Virtualizing storage in the array can bring the advantages of storage virtualization to the data center without the complication of additional appliances. Simplifying migration is one of the biggest reasons to implement storage virtualization, so the appliance approach, which can be used with any storage system, is probably more appropriate for most users.