New horizons in Enterprise Storage: NAS gateway precursors SAN/NAS convergence - Cover story - Industry Overview

Computer Technology Review, Feb, 2003 by Christine Taylor Chudnow

In today's high-speed, ultra-connected data world, IT departments struggle to acquire and manage adequate storage for business applications and user files. This differs enormously from the time-honored practice of throwing on more storage whenever you need it. That approach worked nicely for a while with limited amounts of data and cheap storage capacity, but, today, managing many different devices and huge volumes of data is challenging and expensive. Organizations need a storage strategy that simplifies storage and server management, enables storage to be shared among all attached users, and adapts to meet changing needs. This often means storage area networks, but direct-attached storage (DAS) and network-attached storage (NAS) are very much in the picture. Although some storage vendors offer hybrid devices that combine NAS and SAN, the more common solution is to use NAS gateways. NAS gateways consist of NAS appliances that connect to SAN storage through a Fibre Channel port, providing both file-and block- based data storage that is manageable, flexible and scalable.

NAS and SAN in the Enterprise

As storage needs grew in the enterprise, network-attached storage (NAS) devices, or filers, became a popular alternative to direct-attached and network storage. Well-funded data centers could afford high-end NAS filers from NetApp or EMC, which were scaleable and provided sophisticated management options. But departments, workgroups and smaller divisions bought quantities of economical and simple NAS devices to handle their file storage. NAS was very popular--so popular that it created a giant corporate headache for IT groups charged with centralizing storage operations across the enterprise. And as NAS proliferated within divisions, the local IT departments began to struggle. Many of them were not used to managing big chunks of storage with provisioning and different operating systems, and few NAS devices lent themselves to centralized management.

No one wanted to replace NAS with SAN--the two serve distinct and valuable purposes. But storage administrators struggled with managing both types of storage. They have distinct management interfaces, different (and often incompatible) backup procedures, cannot share capacity, cannot load-balance application workloads across each other, and migrating application storage between environments is challenging.

SAN and NAS are similar in some ways: They are storage devices that combine multiple disks into arrays for availability and performance, and usually have a buffer cache to provide additional performance. NAS is excellent for providing heterogeneous and simultaneous file sharing, but suffers from network protocol overhead: The protocols maintain a large amount of native error-checking and integrity features. The overhead can impact NAS performance. This is not an issue with channels or SANs, so using SANs for larger transfers offers high performance and more predictable response. SANs lack NAS file sharing capabilities.

SANs live on dedicated networks, most commonly Fibre Channel--Fibre Channel provides any-to-any connections between hosts and storage. Like DAS, I/O requests access devices directly. SANs are optimized to handle storage traffic at high speeds with single points of control, and offload significant processing tasks from the LAN. SANs divide the storage up into pieces called logical units, or LUNs (Logical Unit Number). LUNs are made to be manageable:

* They're relatively large so there are typically just a few per zone, which makes LUN management a simpler prospect for zone administrators. For example, a LUN may be one entire array, or a very large array may be divided into two or three LUNs.

* They are similarly sized and can only be accessed in whole blocks, which speeds up data transfer. Using command level interfaces, wizards, or semi-automated procedures, IT administrators map LUNs to blocks on the disk subsystems.

* Block-level data on the LUNs view the SAN as direct-attached storage, making SANs a good choice for database storage.

NAS consists of a specialized processor with its own disk storage. It attaches to the LAN or WAN using specialized file access and sharing protocols, and uses its processor to service file requests instead of allowing direct access to its storage. NAS is optimized for simple management and file sharing across workgroups and platforms, and is a wild and woolly environment compared to the staid SAN. NAS systems differ from SANs because they present their storage as a set of files instead of LUNs:

* Files are more dynamic than LUN's steady data blocks--they come in radically different sizes and are easily created, modified or deleted.

* This dynamic and hierarchical environment supports many thousands of files instead of tens or hundreds of LUNs.

* NAS developers configure the systems to handle sophisticated space management and performance using full file systems, a very different environment than SANs.

Ken Steinhardt, EMC's director of technology analysis, believes that application requirements will ultimately dictate NAS or SAN choices and spending: "NAS is very good at doing many, many, many small things. SAN is inherently better at doing large things very well. It is really at the application level that the functional difference and the use for either will come into play." Since both storage-networking technologies serve application requirements, it makes sense to consolidate them as much as possible. Today's most common convergence technology is a NAS gateway.