In just a few short years storage virtualisation also known as block virtualisation, 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 systems, storage virtualisation soothes a wide range of storage management woes for small and mid-size organisations. 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.

Storage virtualisation 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 virtualisation 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 virtualisation 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, virtualisation 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 virtualisation is nondisruptive data migration. For large organisations, moving data is a near-constant fact of life. As old equipment comes off lease and new gear is brought online, storage virtualisation 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.

Virtualisation can also help you achieve better storage utilisation and faster provisioning. The laborious processes for provisioning LUNs and increasing capacity are greatly simplified — even automated — through virtualisation. 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 utilisation from between 25 and 50 percent to more than 75 percent using storage virtualisation technology.

Four architectural approaches

In a virtualised SAN fabric, there are four ways to deliver storage virtualisation services: in-band appliances, out-of-band appliances, a hybrid approach called split path virtualisation architecture, and controller-based virtualisation. Regardless of architecture, all storage virtualisation 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 virtualisation software will run in an intelligent switch or a purpose built appliance. Providers of split path virtualisation controllers are EMC (Invista), Incipient, and LSI Logic (StoreAge SVM). 

Array controllers have been the most common layer where virtualisation services have been deployed. However, controllers typically have virtualised only the physical disks internal to the storage system. This is changing.  A twist on the old approach is to deploy the virtualisation intelligence on a controller that can virtualise 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 virtualisation is Hitachi Universal Storage Platform.

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