What is NAS and why would it be implemented in a SAN environment?


Network-attached storage is storage that is connected directly to a network such as a LAN that provides file level access to data using standardized protocols such as NFS (Network File System) or CIFS (Common Internet File System).

Computer systems can access data from a NAS over a network via a file "redirector" that changes the access to a file from the native file system (on the originating computer system) to a network operation using TCP (Transmission Control Protocol) to a remote server that is running software to provide the file system to support the individual client access. The remote server with its attached storage is the Network Attached Storage. The file system on the NAS server determines the location of the data requested by the application client whether it is in its cache or on the storage.

A simpler way to describe NAS when contrasted against Direct-attached storage (DAS) or storage area networks (SAN) is that NAS does "file-level I/O" while DAS and SAN do "block-level I/O." The file I/O done by NAS means that the NAS device is given a "File Handle:Offset" request for specific information in the file and the NAS device has the file system to be able to process that and resolve it to a specific block on a disk device. The NAS device will use block I/O to access the disk device. For DAS and SAN, block level I/O is used for all operations.

NAS is very useful for sharing files because of the file access semantics built into the protocols and because of its simplicity in installation and administration. There are valid reasons both from application requirements and from administrative issues to have SAN attached storage and NAS.

Randy Kerns
Evaluator Group, Inc.


This was first published in February 2003

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