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 of course 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.
A split path system leverages the port-level processing capabilities of an intelligent switch to offload the metadata and control information from the data path. Unlike an out-of-band appliance, in which the paths are split at the host, split path systems split the data and the control paths in the network at the intelligent device. Split path systems forward the metadata and control information to an out-of-band compute engine for processing and pass the data path information on to the storage device. Thus, split path systems eliminate the need for host-level agents.
Typically, split path virtualization software will run in an intelligent switch or a purpose built appliance. Providers of split path virtualization controllers are EMC (Invista), Incipient, and LSI Logic (StoreAge SVM).
Array controllers have been the most common layer where virtualization services have been deployed. However, controllers typically have virtualized only the physical disks internal to the storage system. This is changing. A twist on the old approach is to deploy the virtualization intelligence on a controller that can virtualize both internal and external storage. Like the in-band appliance approach, the controller processes all three paths: data, control, and metadata. The primary example of this new style of controller-based virtualization is Hitachi Universal Storage Platform.