Storage area network

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A Functional Example of a SAN - storing functions in block mode

In traditional computing, a storage area network (SAN) is a network designed to attach computer storage devices such as disk array controllers and tape libraries to servers. The primary characteristic of a SAN versus that of network attached storage (NAS) is the ability to read and write data over a network at the block level instead of the file level. This, along with robust network construction, generally means that a SAN offers better performance than a NAS for certain I/O intensive applications. This performance increase comes with a corresponding increase in cost and complexity.

Further Defining A SAN

A SAN allows a machine to connect to remote targets such as disks and tape drives on a network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached devices.

A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust. The term SAN is usually (but not necessarily) identified with block I/O services rather than file access services.

Storage Area Network Protocol Usage

The performance that provided an adequate SAN experience for users was originally only attained through Fibre Channel technology. Using the Fibre Channel protocol stack, servers would have two interfaces on them, one for IP and serving user requests and another for the Fibre Channel interfaces.

Fibre Channel has the market advantage in the SAN arena; it is a proven technology with excellent performance characteristics. While Fibre Channel expertise is not as easy to find as IP expertise, the Fibre Channel market has proven itself to organizations as a stable, fast, storage environment.

As Ethernet speeds increased, a new standard was introduced. Known as Internet SCSI (iSCSI), this standard is the encapsulation of the SCSI interface protocol in to IP packets. This arrangement allows the same high performance SCSI protocol used in direct attached storage (DAS) to be used anywhere there is IP connectivity. There is especially true over a wide area network (WAN).

File versus Block Level Access

The function of a SAN is very similar to that of NAS. This distinction is further blurred in the popular press, where the two terms are often used interchangeably, despite their very different technical underpinnings. (It’s been easier for the market to adopt to a sloppy differentiation than to be technically accurate. It hasn’t hurt the consultants either.) The distinguishing characteristic between the two services, however, is that the NAS operates at the file level while a SAN accesses data at the block level.

File level access means that the remote host operating system views the NAS resources as files. The NAS client on the host treats the share as a remotely mounted file system; there is never the impression that the NAS file system is in any way local to the client host. This means that the client host only views the file structure on the NAS server, not the actual details of the file system. Consider the NAS diagram above. The client host will send a request to one of the NAS servers: “Let me see what files are available so that I can display the presence of these files locally.” If a host requests a document or spreadsheet from the NAS, the file is transferred in its entirety as a series of packets. When the client changes the document, the file as a whole is transferred back to the NAS.

The file-based operation of a NAS makes it the ideal choice for applications that read or write infrequently but would still benefit from centralized storage advantages in backup and management costs. For example, if a law office uses several standardized forms to create contracts, and it would like to keep completed contracts in a central location for partners to access and for backup, a NAS would be the ideal solution.

Block level access typically describes how DAS operates. Information is transferred between the system and the direct attached storage as a series of information blocks. This is the same way a SAN operates; a network host views a remote storage server as a locally attached drive and transfers information in blocks as if it were accessing a local hard drive. To understand how this works in contrast with a NAS, imagine an application operation that requires a great deal of reading and writing of information (e.g., using a database). Unlike a NAS, the client system would not access the whole database, but it could read and write information to the database in small chunks as it would if the database existed on a locally attached drive.

Block level operation means that a SAN is the ideal solution when used with database applications and other applications requiring many small read/write operations (e.g., an Email server). A company with a large integrated messaging system, such as Microsoft’s Exchange server, would benefit from a SAN since each user’s email files are part of a much larger .pst file. This application would benefit from the faster block level access of the SAN and the ability to read and write small sections of the personal email file without having to transfer the entire object each time the user needed to check email.

External Links

An extensive introduction to storage area networks Search Storage at TechTarget

See Also

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2007q1/nas-san.mp3%7Cdownload</mp3> | NAS vs. SAN