NAS and SAN: putting it all together - Storage Automation

Computer Technology Review, Feb, 2002 by Christine Chudnow

I was at a conference when a reseller approached me. "Can you please, he said wearily, "Explain the difference between NAS and SAN to me?" He paused. "In twenty-five words or less?"

I didn't blame him for asking. Definitions of networked attached storage and storage area networks are as fluid as water in some vendor camps, and as difficult to quantify. But an overriding definition is that network attached storage serves file-based data, while storage area networks stores block-based data. There are exceptions to the rule, but it's a vital distinction.

All data contains block addresses, but different applications prefer to receive their data in either block or file formats. The advantage of block-based transfers is speed, since blocks exist at the lower levels of the OS and sit close to the arrays. Fibre Channel excels in block-based transport and forms the backbone of most SANs, which use block based protocols such as SCSI-3. File-based transfers use protocols like CIFS and NFS over IP networks. When an application issues a request for a file, the file server assembles related blocks and packetizes the resulting file for LAN transmission. File-based transfers offer more high level functionality, such as the ability to access the same file via UNIX or Windows.

The problem is that few servers can effectively use both file-based or block-based delivery protocols. But if NAS and SAN could successfully converge into single devices and/or integrated networks, customers would benefit by reducing the sheer number of devices they must buy, integrate, and manage.

Some vendors integrate NAS and SAN by using a NAS front-end and SAN back-end--examples include EMC's CLARiiON coupled with a Symmetrix back-end, and Procom's NetForce 3200C with an attached high-end tape library. However, newer convergence technologies include both device-level and switch-based approaches. A device-level player is LeftHand Networks, which makes LAN-based storage arrays that handle either block-based or file-based data. At present the user must configure an array to handle one or the other data type, but the product's next generation arrays will handle both. This is not necessarily an advantage--in order to accomplish dual tasks, recognizing the nature of the incoming data stream slows the device's performance.

Pirus Networks champions switch-based storage management in the SAN. Their intelligent switch operates in Fibre Channel SANs and shunts both file- and block-based data to shared storage pools. The switch recognizes the application's request for file-based or block-based data, and allows appropriate access to the SAN's storage devices. In both cases storage locations are transparent to users, and administrators may control devices and allocation from single consoles.

Successful NAS/SAN convergence will affect data management and TCO by offering scalable storage pools, reduced restore and backup times with new kinds of secondary storage media, and greater data manageability with central control points. Michael Alvarado, Global Data Fabric Solutions Marketing Manager for Network Appliance, lists three user-driven convergence factors:

Combined best features. NAS devices offer plug-and-play capability on standardized network architectures, relieve server I/O bottlenecks by offloading storage and file server functions from application servers, and enable file sharing between multiple platforms. SANs are high performance, dedicated storage networks that interconnect storage systems, backup devices, and servers. Combining NAS standard networking protocols and file sharing with SAN speed and scalability would result in efficient storage environments minus existing drawbacks.

Improved return on technology investment. Storage and networking investments can be very costly, and companies badly want to improve their ROI as high as the five-year mark. Strategies include leveraging existing infrastructure investments, such as using convergence to extend the useful life of storage networking devices.

Stable environments. The last thing a company wants to do is junk their existing infrastructure in favor of entirely new deployments. Converged SAN and NAS architectures can help to leverage existing devices and networks in the midst of new implementations.

Challenges

There are serious technological and business challenges to successfully combining NAS and SAN. TWo of the major challenges arc proprietary protocols and lack of standardization, and a definable customer value proposition

Proprietary protocols and lack of standardization. SAN installations arc lagging way behind the optimistic forecasts of the 90's, as many companies failed to deploy storage area networks because of SAN ongoing cost and complexity. Many companies have gone with NAS instead, but most of these devices do not handle block level storage well, lack a single point of management, and suffer from network latency issues. (A NAS is only as fast as network latency is low, and requires a fast Ethernet connection for acceptable performance.)