Most of us are familiar with the International Organization for Standardization's (ISO's) Open System Interconnection (OSI) network model. That seven-layer cake of applications, protocols, transports, interfaces, and hardware is standard material in almost every text about networking. Many products span two or more layers in the OSI network model, which blurs the definition somewhat. However, the OSI network model provides a framework for technology discussions about networking issues.

Storage networking builds upon many principles of traditional networking, but it adds specific functions and emphasizes different services. Several groups in the storage industry have begun to build shared storage models and to publish and discuss the results—among them are the Storage Networking Industry Association (SNIA) and the Fibre Channel Industry Association (FCIA). SNIA's Shared-Storage Model team has begun to incorporate elements of the FCIA's model into the SNIA model to unify the view.

Obviously, in an area as dynamic as storage networking, any architectural model is a work in progress. The model is meant to capture the functional layers and properties of networked storage systems. Such a model isn't a specification for a "right" or "wrong" way to design a particular network architecture nor an endorsement or recommendation of any product or installation method over another.

Open Storage Model
SNIA touts "open" storage—based on industry standards that let one vendor's product work with another's interchangeably. From a customer perspective, this approach increases product choice, allows for incremental component upgrade, and provides protection against obsolescence. Customers' storage networks can grow incrementally and to any capacity. From a vendor's point of view, this approach makes it easier to bring a product to market. However, standardization tends to blunt significant advantages that one vendor's product might have over another's. The ideal situation is to make products open, but different.

To be open, storage networks rely on well-defined interfaces, interface protocols, and access protocols, all of which require published standards supported by real products—exactly SNIA's goal. The model divides networked storage into hardware, APIs, and protocols. Hardware includes Ethernet, fibre channel, and Infiniband; APIs are for blocks, files, or stored objects; and protocols include Fibre Channel Common Transport (FC-CT), TCP/IP, and others.

The SNIA Shared-Storage Model
The SNIA model breaks network storage into three subsystems: the file/record system, the block subsystem, and the services subsystem.

The model's file/record subsystem provides for overlap between the file system and a database (metadata), which sits between an application on the top and data stored in blocks on devices. Several possible access paths lead from an application through the file subsystem and block aggregation layer to the storage device. The file subsystem provides access methods, packaging (naming and space allocation), and caching (for performance and coherency in distributed systems).

The SNIA shared-storage model isn't device centric and therefore doesn't indicate Network Attached Storage (NAS) systems, Storage Attached Networks (SANs), switches, or other devices. The block subsystem holds the storage devices: disks, tape drives, and solid state devices that store data. At that level, you find space management, block aggregation, striping, and redundancy (mirroring) features, and address mapping. The block level also includes a caching function. The block subsystem determines where operations take place: on a host (logical volume, host bus adapter—HBA, or device driver), on a storage device (array controller or disk controller), or over the SAN using an HBA or a specialized appliance. Data is moved in and out of the block system as a vector of blocks.

All of the pieces of the services subsystem remain to be more fully described and worked into this model. In fact, many of the principles of discovery and monitoring, for example, are yet to be fully worked out in the industry itself.

The Importance of a Shared-Storage Model
Creating a shared-storage model isn't just an exercise in philosophy. By establishing an architecture that accommodates common shared storage products as a set of layers and services, you can expose which services are provided and where, what future areas of interoperability must be explored, and what key pros and cons of different storage architectures will affect decision making.

SNIA is in the interoperability business, and the shared-storage model offers varied industry participants a common language. (By the way, the SNIA Web site offers a glossary of common storage terms used throughout the industry.) Importantly, the model helps vendors better explain to customers the differences between products and helps customers understand the tradeoffs that they make installing one type of storage system or networking component rather than another.

Click here to view the SNIA Shared-Storage Network model and scroll to page 11.