Product Guide: SAN and NAS virtualisation

In just a few short years, storage virtualization, also known as block virtualization, has proven its worth in the large enterprise and traveled that well-worn path from pricey boutique solution to affordable commodity. As a standard feature in all but the most modest mid-tier storage arrays, storage virtualization soothes a wide range of storage management woes for small and mid-size organizations. At the same time, dedicated solutions from top-tier vendors deliver the greatest ROI to large shops managing large SANs with intense data availability requirements.

Computerworld feature Virtualization 101: What is virtualization?

Storage virtualiztion creates an abstraction layer between host and physical storage that masks the idiosyncrasies of individual storage devices. When implemented in a SAN, it provides a single management point for all block-level storage. To put it simply, storage virtualization pools physical storage from multiple, heterogeneous network storage devices and presents a set of virtual storage volumes for hosts to use.

In addition to creating storage pools composed of physical disks from different arrays, storage virtualization provides a wide range of services, delivered in a consistent way. These stretch from basic volume management, including LUN (logical unit number) masking, concatenation, and volume grouping and striping, to thin provisioning, automatic volume expansion, and automated data migration, to data protection and disaster recovery functionality, including snapshots and mirroring. In short, virtualization solutions can be used as a central control point for enforcing storage management policies and achieving higher SLAs.

Perhaps the most important service enabled by block-level virtualization is nondisruptive data migration. For large organizations, moving data is a near-constant fact of life. As old equipment comes off lease and new gear is brought online, storage virtualization enables the migration of block-level data from one device to another without an outage. Storage administrators are free to perform routine maintenance or replace aging arrays without interfering with applications and users. Production systems keep chugging along.

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.

File virtualization

Just as block virtualization simplifies SAN management, file virtualization eliminates much of the complexity and limitations associated with enterprise NAS systems. We all recognize that the volume of unstructured data is exploding, and that IT has little visibility into or control over that data. File virtualization offers an answer.

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