A new breed: IP SANs show great promise for networked storage - Storage Networking

Computer Technology Review, Oct, 2002 by Mike Strickland

Internet protocol (IP)-based storage has gained significant traction over the course of the past year. The activities of the Internet Engineering Task Force (IETF) and coverage in a wide variety of industry trade publications have not only produced a more refined and detailed vision for IP storage but have also added much greater visibility. Clearly, more work needs to be done but exceptional progress has been made.

There is mounting evidence that IP-based storage area networks (SANs) have what it takes to become a viable alternative, as well as a complement, to Fibre Channel-based SANs.

For one thing, it's hard to discount the value of only having to support one networking technology for both the LAN and the SAN, rather than the additional, specialized IT headcount required for supporting Fibre Channel for the SAN and Ethernet for the LAN. So, while the business case is clear, some important technical issues are still under discussion.

For example, among the various approaches for IP storage that have been discussed by the IETF are FCIP (Fibre Channel over Internet protocol) which entails tunneling Fibre Channel traffic over TCP/IP networks, as well as iSCSI, which involves encapsulating SCSI over TCP/IP networks. The advantage of the iSCSI approach is that it leverages the ubiquity of mature technologies-SCSI and IP-and also creates a seamless link between an Ethernet LAN and a SAN in the same enterprise.

If IP storage is to make good on its potential, it has to meet very particular performance, flexibility, scalability, and data integrity requirements. And as the makers of storage routers, subsystems and host bus adapters scramble to design and build products to serve the burgeoning IP storage market, they must determine what firmware and silicon products are suited to meet the unique requirements of this new approach to building SANs.

A key to satisfying those unique performance, flexibility, scalability and data integrity requirements comes in the form of a new kind of processor--a storage network access processor--that is crafted precisely to satisfy those requirements.

Performance

On the performance front, an optimal IP SAN must have the ability to keep pace with network speeds for all kinds of storage traffic. At present, network speeds run at 1Gbps and will likely move beyond 10Gbps. Another performance-centric issue comes in the form of making sure to provide for maximum offload of host processors, for example a server CPU. After all, any network protocol processing that consumes CPU cycles interferes with application processing, the primary job of the CPU. In an optimal IP SAN, the proper offloading of a host processor is achieved when a storage network access processor (SNAP) autonomously handles the complete TCP/IP traffic load-including fast path and exceptions--and ULPs such as iSCSI. A SNAP also provides for minimum interrupts to the host processor and direct data placement into host buffers.

Flexibility

In terms of flexibility, SNAPs need to be adaptable to continuously evolving network protocols. But beyond that, the design of the SNAP should give storage networking QEMs the luxury of being able to ignore connectivity issues (because they've already been addressed) and to focus their efforts on adding value through storage services. In this context, connectivity refers to all of the issues associated with connecting devices to one another.

Scalability

When it comes to scalability, the SNAP should be designed in such a way that it can keep up with line speeds of 1Gbps and can deal with an online transaction processing (OLTP) profile and the smaller block sizes that such a profile implies. Beyond that, this new kind of processor must be capable of supporting a large number of simultaneous TCP/IP connections. For example, in a typical SAN a component like a host bus adapter (HBA) may only need to support 1,000 to 2,000 connections but other parts of that same SAN may need to support tens of thousands of connections. A well-designed SNAP should not place a limit on the number of connections.

Data Integrity

Data integrity is the final key measure of a premier IP SAN implementation. Since traditional network processors can rely on higher protocol layers to handle any data integrity problems, such as memory soft errors, they don't need to implement data-integrity mechanisms themselves. But a SNAP operates not only on the network protocol layers but also on the upper layer protocol. Therefore, it cannot depend solely on the network layer to provide data robustness. As a result, the SNAP needs to implement comprehensive data protection mechanisms.

Non-SNAP Alternatives

While IP SANs have very specific requirements, it's important to note that a variety of earlier-generation processors--namely general-purpose processors, network processor units (NPUs), and TCP/IP offload engines (TOEs)--have been used for protocol processing up to this point. It's worthwhile to look at these three approaches to protocol processing.