Protocol work melds storage methods

Sharing data across multiple clients on a storage-area network (SAN) is easier said than done. As a result, the File System Working Group (FSWG) of the Storage Networking Industry Association is developing a proposal to bring the data-sharing capabilities of network-attached storage (NAS) devices to SAN environments.

NAS devices provide alternative storage for authorized clients on a LAN. Also known as storage appliances or multiprotocol filers, NAS devices are popular because the storage needs of an organization often outpace the capacities of existing systems.

NAS appliances interface with LAN clients through a file-sharing protocol, such as Common Internet File Services (CIFS) for Microsoft, Network File System for Unix or NetWare Core Protocol. The appliances provide the client with directory services, security and the appropriate view and format of the client's data. The NAS device uses an internal operating system, which acts as a control mechanism for managing data storage, semantics and control for reading and writing data onto its disks.

A SAN is a more specialized, alternative strategy for managing growing storage requirements. A SAN is a separate high-speed network (typically Fibre Channel) that connects multiple clients to a high-speed, high-capacity disk array.

SAN-enabled clients use the disk array instead of their internal disk subsystems. However, because SAN storage provides device-level sharing, data stored on the SAN is accessible only by the specific client storing the data. Bringing file sharing to a pure SAN configuration requires the implementation of a heterogeneous clustered file system. Such a system is currently unavailable because its high degree of technical complexity makes it prohibitive from cost and performance standpoints.

On the other hand, merging existing NAS and SAN technologies is a highly realistic SAN file-sharing proposal. Several vendors have already introduced proprietary solutions.

Protocol extensions proposed by the FSWG enable file sharing through the ability to create hybrid NAS/SAN architectures. Such a configuration connects clients to a hybrid NAS/SAN server via TCP/IP and to the SAN disk array via the Fibre Channel Protocol. In such a model, a hybrid NAS/SAN server manages multiuser control information for SAN-stored data, serving as a traffic manager for disk reads and writes, preserving cache coherency for multiuser access and enforcing security semantics. Once the hybrid NAS/SAN server grants access permission, clients connect directly to the SAN disk array for high-speed file data transfer.

Compared with a conventional SAN, the hybrid NAS/SAN server's data control functions involve additional data transfers of data control information. However, the control data exchanges inherently produce low overhead, involving the metadata portions of files. Furthermore, the additional overhead is well justified by the architecture's ability to provide SAN data sharing.

As for dealing with instances in which more than one user wants access to the same data simultaneously, vendors can use the CIFS protocol's soft locking mechanism to manage file system coherency and deal with multiuser contention. The soft lock operates as a file or record lock that can be relinquished for contention-resolution purposes. For example, if one client wishes to write to the same file as a second client, the NAS/SAN server revokes the soft lock given to the first client and takes over the management of write operations using traditional NAS methodologies.

The FSWG's protocol-extension recommendations include the use of a soft lock, as well as four other add-on capabilities to coordinate reads and/or writes: disabling NAS-controlled data; preallocating storage; modifying independent and explicit metadata; and identifying disk block allocations (also known as extent lists).

Extending these file-sharing protocols will let vendors deliver SANs that can share data with any authorized client on the network. If clients have a traditional SAN connection, they will retain the advantages of high-speed direct-disk access. If there is contention between SAN clients, NAS-style cache coherency is invoked. If clients are only LAN-connected, they will use the NAS as a conduit for SAN-maintained information.

The hybrid model provides IT with a greater degree of flexibility in planning and justifying SAN implementations, while also facilitating multivendor interoperability.

Smith is the director of marketing at CrosStor Software, a maker of storage software and storage-centric operating systems in South Plainfield, N.J. She can be reached at sue@ crosstor.com.

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