In a SAN without virtualization, storage devices are connected directly to host systems and are maintained locally by those host systems.
Although storage area networks (SANs) have introduced the principle of networks, storage devices are still mainly assigned to individual host systems and storage is still mainly created and maintained at the RAID subsystem level. Therefore, RAID controllers of different types require access to both the hardware and the software that is used.
Virtualization provides a complete change from the traditional storage management. It provides a central point of control for disk creation and management, and therefore requires changes to the way in which storage management is done.
Fabric level virtualization is the principle in which a pool of storage is created from more than one disk subsystem. This pool is then used to set up virtual disks (VDisks) that are made visible to the host systems. These VDisks use whatever storage is available and permit a common way to manage SAN storage.
Fabric level virtualization can be done in either of two ways: asymmetric or symmetric.
With asymmetric virtualization, the virtualization engine is outside the data path. It provides a metadata server that contains all the mapping and the locking tables. The storage devices contain only data.
Because the flow of control is separated from the flow of data, input/output (I/O) operations can use the full bandwidth of the SAN. A separate network or SAN link is used for control.