This type of storage virtualization can operate in two distinctly different ways: in band and out of band. In-band devices are generally implemented within the storage switches or appliances that sit in between hosts and the storage devices. These devices maintain the necessary metadata to map virtual disk blocks to the physical devices they actually reside on and actively proxy I/O operations between the two. This can add a small amount of latency as an additional "hop" is inserted between the host and the physical storage it is attempting to access. However, in-band storage appliances often implement their own storage cache to counterbalance this effect.
Out-of-band storage virtualization generally involves the use of a metadata server combined with software loaded onto each host that needs to access virtualized storage. Before the host attempts to access storage, it first performs a lookup against the metadata server to learn where the storage is located. Then the host is free to talk directly to the storage device without an intermediary. Though this approach avoids much of the latency that can be introduced by in-band implementations, it also prevents additional caching beyond what the physical storage device might already implement.
By placing this abstraction layer in front of multiple storage devices, network-based storage devices give you complete freedom to move the data making up a virtual disk from one SAN to another without the server having any idea that the move is taking place. Likewise, they also open the door to not only cross-vendor storage replication -- generally an impossibility due to the proprietary nature of array-based replication -- but also transparent storage failover. This kind of functionality can prove to be invaluable in extremely large storage environments where a wide range of storage devices are in use and downtime is not an option.
This article, "What storage virtualization really means," originally appeared at InfoWorld.com. Read more of Matt Prigge's Information Overload blog and follow the latest developments in storage at InfoWorld.com.