The shared storage vision

A major objective of the storage networking movement within the computer industry is to address the needs of corporate computing environments for storage systems that offer the scale, flexibility, availability, and other attributes that are typically required by enterprise networks.

Storage is increasingly recognized as a distinct resource one that is best thought of separately from the computer systems (hosts) that are its consumers and beneficiaries. Such storage is increasingly often shared by mul tiple hosts and acquired and managed independently from them. This is in contrast to the historical view (host-attached storage) that storage is an intrinsic part of a computer system, i.e., a "peripheral."

This trend towards shared storage recognizes the critical value of the information entrusted to the storage system, as well as the fact that storage represents a significant portion of the investment in a typical computing environment. In turn, the trend offers the IT community several benefits, which can be summarized as:

• increased flexibility in deploying and managing storage,

• improved quality of service, and

• increased operational efficiency.

These benefits are each highly valued in today's businesses, which must react quickly to changing opportunities while meeting rapidly rising expectations for service quality and yet are constrained by the need to accomplish more with limited resources.

The key enabling technology for shared storage is networking technology that can provide high bandwidth, large scale, good connectivity, and long-distance connectivity at a cost that makes shared storage an attractive alternative to the historical host-attached storage model.

We use the term storage network or the acronym "SN" to refer to this concept throughout this document. We have intentionally avoided the more commonly used "SAN" and "NAS" because of their associated connotations, and because we wish to de-emphasize any particular networking technology. A storage network, in our usage, could be Fibre Channel, ESCON, Ethernet, etc., so long as its primary use is for accessing the shared storage.

Although shared storage environments can bring many benefits, they also have a number of challenges. The Storage Network Industry Associa tion (SNIA) was formed to communicate the benefits of this new paradigm, and to provide a forum for computer vendors, storage vendors, and the IT community to address its challenges together. The primary purpose of the SNIA Shared Storage Model is to provide a language for identifying and articulating those challenges in order to facilitate the creation of effective answers that in turn make possible the full benefits of the shared storage vision.

Why shared storage?

Through much of the history of computing, storage has been seen as an intrinsic part of computer systems. While storage once was regarded as a "peripheral," more recently it has come to be thought of as a storage subsystem, but still uniquely associated with a computer. The principal exceptions to this have been mainframe computer complexes and computer clusters where a modest number of cooperating computer systems share a common set of storage devices.

As business has become more dependent upon computing, it has also become more dependent upon data, the life-blood of computing. While a failed processor can usually be readily replaced and operations continued almost immediately after the replacement, a failed storage resource requires replacement followed by typically time-consuming restoration of data, all too often with some loss of recent changes to that data that requires recovery action, before operations can continue. As a result, storage and the disciplines of caring for data and the storage on which it resides have gown in visibility and importance.

In addition, the fraction of the purchase price of a computer system that is represented by the storage component has grown over time to the point that now the storage component of a computer system is often in the vicinity of half of the total price.

Beyond the purchase price of storage, the total cost of owning storage has become a significant part of the cost of maintaining the computing environment. In other words, the acquisition cost is a small portion of the total cost of ownership of storage over its lifetime.

Computing environments have grown as business has become increasingly reliant on computing. As this has happened, computer systems have grown in size and in number. Because the traditional computing model associates storage uniquely with a computer system, a computing environment with many computer systems has many storage and storage management environments to maintain and operate one per computer system.

In responding to these trends, the IT community has come to view storage as a resource that should be purchased and managed independently of the computer systems that it serves. The IT community has also increasingly come to view storage as a resource that should be shared among computer systems. These changes allow more focused attention on storage, which is expected to lead to reduced costs and higher levels of service, and more flexibility through the sharing of the storage resource. This, in turn, allows IT professionals to provide improved quality and response time as business needs change.

As the storage system for a computing environment becomes a shared, independent resource, additional requirements emerge:

• Reliability as required of any large, shared, critical resource

• Scalability to match the size, performance, and physical and geographic placement of computing environments

• Manageability to provide high levels of service, and achieve the expected reduction in operational expenses

• Standards-based interoperation to avoid excessive vendor dependence in a large, critical component of data centers

Another key trend is the evolution in communications technology that has made available high performance communication between many nodes over distances ranging from a few inches to fully global systems at reasonable cost. (Despite this, high performance over large distances is still relatively expensive compared to local connectivity, and it is subject to speed-of-light related delays that can be significant in comparison to storage device access times.)

When all of this is considered, a structure emerges that achieves the goals of reliability, scalability, and manageability. That structure is a storage system comprising many computer systems that are the consumers of the storage system, many storage devices, and extensive management capabilities all interconnected richly and with the necessary high performance. This is the shared storage environment.

The potential

Shared storage is a powerful concept, bringing together into single storage systems essentially unbounded collections of interconnected and geographically unconstrained storage resources and management capabilities.

This redefines storage, where traditional storage systems are very limited (both in scope and extent) and where management capabilities are typically distinct from storage systems.

The power of the shared storage environment lies in a richly interconnected set of resources and in defining the storage environment as a storage system in its own right. The former opens many avenues for exploiting connectivity. The latter enables the storage system to be the focus of invention, deployment, and operation as an entity independent of the hosts that it serves. Many aspects of ongoing activity under the "storage networking" umbrella support these claims; it is likely that more are to come given the high level of energy being applied to this field.

Some examples of potential benefits are:

• Reduced Total Cost of Ownership (TCO) of storage solutions through intelligent Storage Resource Management. For example, if three servers share a pool of storage, they need not each have spare storage to accommodate growth but can pull from a common pool. They also may share a backup resource that each can use.

• Better performance or efficiency through use of network enabled storage management approaches, i.e., "server independent" storage management, consolidated storage management, data movers, 3rd party copy, etc.