For companies where demands for storage are doubling every year, the traditional approach of discretely allocating specific storage devices to each application server is impractical.
Not only is scalability limited, but reliability and performance considerations suggest that storage should not be confined to one or two servers but instead become a network resource that can be dynamically shared by multiple servers and applications. And networked storage allows the distance between servers and media to be extended, improving the physical security of data centres.
However compelling, these benefits of networked storage are only a starting point. When storage becomes a network resource, it can assume an unprecedented range of capabilities, including server independence from media format, dynamic re-allocation of storage volumes to server hosts (without service interruptions), and volume mirroring both locally and remotely.
Thanks to these new capabilities, enterprise storage is freed of some of its physical restrictions and becomes a plastic container that can be quickly and easily shaped to satisfy changing business requirements. Welcome to storage virtualisation, and to its promise of more flexible and less expensive management of networked storage.
Transcending physical storage
The goal behind storage virtualisation is to isolate the server OS and its applications from the technicalities of storage devices, thereby simplifying media management and the allocation of storage space to each server - or host - computer. Normally a host computer must manage specific storage devices with certain capacities and other technical characteristics. With storage virtualisation software in place, servers and their applications use logical volumes that shield the host from the complexities of physical devices.
A good analogy to storage virtualisation is RAID (Redundant Array of Independent Disks). RAID solutions hide the technicalities of the media, such as cylinder, track, and block geometry of a disk, and present the OS with logical volumes. Storage virtualiSation delivers similar functionality, but on a broad scale. Whereas RAID is limited to a narrow range of storage devices attached to one host, storage virtualisation extends the benefits of logical volumes to the entire storage network.
The benefits of logical volumes are many. Without storage virtualisation the host OS becomes strictly dependent on the technical characteristics of the storage devices, so that apparently simple activities - such as replacing or adding media - require stopping the host and its applications while setting up new storage devices and copying data. By contrast, storage virtualisation removes most media management problems from host computers and gives administrators the tools to create and connect logical volumes to application hosts, regardless of the technical specifications or network locations of the physical storage devices involved.
A storage virtualisation system can create a logical volume using a segment from a physical volume and can improve performance or data reliability by aggregating multiple disks or segments. To further ensure data reliability, logical volumes can be mirrored either locally or remotely. Regardless of the logical volume's physical composition, the host computer sees only an available data container with a certain capacity; all other technicalities are handled elsewhere using the virtualisation software as the interface between host and media.
The obvious advantage of storage virtualisation is simplified and less expensive storage administration. It is not surprising that most storage vendors offer virtualisation capabilities, although significant differences exist among solutions. A very apparent distinction is the location of the storage virtualisation logic, which can reside on host computers, on storage devices or on network devices such as a switch, a router, or a dedicated computer.
Moving beyond abstraction
These apparently contradictory approaches derive from an architectural choice - often driven by the nature of the vendor's networked storage products - that dictates where in the storage network is more convenient to locate storage management software and hence virtualisation logic. Currently there are no standards in place to guide the implementation of storage virtualisation. This, unfortunately, creates the battleground for animated and often confusing debates, and makes purchasing decisions extremely difficult.
No company makes technological choices in a vacuum; all must find solutions that integrate with the existing infrastructure at affordable cost. The variety of virtualisation offerings from vendors such as Datacore Software, Softek, StorageTek, Veritas, Vicom and Xiotech provides companies with a wider range of options to meet their increasing storage requirements without disrupting their existing platforms.
From a broader perspective, storage virtualisation could be the "killer application" of networked storage and become one of the crucial factors driving a company's choice of storage vendor. Recent developments in this already active market suggest that storage vendors are moving the focus of competition to this area.
For example, Hewlett-Packard's recent acquisition of StorageApps suggests that the company will reinforce its Federated Storage Management Architecture with the storage virtualiszation technology of StorageApps.
EMC, considered by many to be the market leader in networked storage, has announced its intention to extend the storage virtualisation capabilities of its Clariion and Symmetrix lines with networkcentric, device-independent functionality that will include simplified administration via companywide policies and automated storage reallocation to meet performance objectives.
Similar indications come from Compaq's revised ENSA-2 (Enterprise Network Storage Architecture), which projects storage management and virtualisation software such as SanWorks into a vision of global storage networks. By contrast, Hitachi Data Systems is pursuing a completely different approach, promising interoperability between its hardware offerings and management solutions from vendors such as Veritas and McData, and offering its customers remote management of their networked storage.
It is difficult to resist making the analogy between networked storage and networked computers, as they both serve the similar purpose of connecting an application to its data repository. At the dawn of the information era, network operating systems such as Banyan Vines and Novell NetWare were instrumental to the popularity of network computing because they offered companies undeniable financial and practical advantages over the discrete administration of data residing on PCs, despite the substantial cost of networking equipment.
Storage virtualisation is likely to play a similar role in the adoption of networked storage, by offsetting high hardware acquisition costs with otherwise unattainable functionality and administrative efficiency.
The bottom line
Storage virtualisation is the logical complement to networked storage. By transforming a network of storage devices into a virtual pool that can be shared among applications, storage virtualisation solutions can deliver substantial financial and practical benefits to companies facing a steep increase in storage requirements.
Storage virtualisation technology is still evolving. Companies can achieve immediate benefits, but must be careful when selecting from the vast range of available solutions to choose those that are compatible with existing investments and free of proprietary restrictions.
This was first published in October 2001