Virtualization can also help you achieve better storage utilization and faster provisioning. The laborious processes for provisioning LUNs and increasing capacity are greatly simplified -- even automated -- through virtualization. When provisioning takes 30 minutes instead of six hours and capacity can be reallocated almost on the fly, you can make much more efficient use of storage hardware. Some shops have increased their storage utilization from between 25 and 50 percent to more than 75 percent using storage virtualization technology.
Four architectural approaches In a virtualized SAN fabric, there are four ways to deliver storage virtualization services: in-band appliances, out of-band appliances, a hybrid approach called split path virtualization architecture, and controller-based virtualization. Regardless of architecture, all storage virtualization solutions must do three essential things: maintain a map of virtual disks and physical storage, as well as other configuration metadata; execute commands for configuration changes and storage management tasks; and transmit data between hosts and storage.
The four architectures differ in the way they handle these three separate paths or streams -- the metadata, control, and data paths -- in the I/O fabric. The differences hold implications for performance and scalability. An in-band appliance processes the metadata, control, and data path information all in a single device. In other words, the metadata management and control functions share the data path. This represents a potential bottleneck in a busy SAN, because all host requests must flow through a single control point.
In-band appliance vendors have addressed this potential scalability issue by adding advanced clustering and caching capabilities to their products. Many of these vendors can point to large enterprise SAN deployments that showcase their solution's scalability and performance. Examples of the in-band approach include DataCore SANsymphony, FalconStor IPStor, and IBM SAN Volume Controller.
An out-of-band appliance pulls the metadata management and control operations out of the data path, offloading these to a separate compute engine. The hitch is that software agents must be installed on each host. The job of the agent is to pluck the metadata and control requests from the data stream and forward them to the out-of-band appliance for processing, freeing the host to focus exclusively on transferring data to and from storage. The sole provider of an out-of-band appliance is LSI Logic, whose StoreAge product can be adapted to both out-of-band or split path usage.