Storage Area Networks For Dummies
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The technical description of a storage area network (SAN) is a collection of computers and storage devices, connected over a high-speed optical network and dedicated to the task of storing and protecting data.

In a nutshell, you use a SAN to store and protect data.

Today, that high-speed network usually consists of optical fiber cables and switches that use light waves to transmit data with a connection protocol known as Fibre Channel. (A protocol is a set of rules used by the computer devices to define a common communication language.) More and more, regular Internet provider (IP)-based networks, such as the Internet, are being used as the network part of a SAN.

The act of using a network to create a shared pool of storage devices is what makes a SAN different. The network is used to move data among the various storage devices, allows sharing data between different network servers, and provides a fast connection medium for data backup and restoration and data archiving and retrieval. Devices in a SAN are usually bunched closely together in a single room, but the network allows the devices to be connected over long distances. The ability to spread everything out over long distances makes a SAN very useful to large companies with many offices.

Fiber versus fibre

Networking geeks chose to use the word fibre to describe the fiber (notice the reversal of the er portion of the word fiber) optic cables used in a SAN. They did this differentiate them from the optical cables used in other networks, such as the Internet. Also, the word fibre is used is because SAN devices use a different language to communicate with each other than do the devices in other networks. The protocol used in a SAN is called Fibre Channel.

All network protocols are divided into layers, like a layer cake. All the layers in the cake are logically tied together into a stack. Each layer of the stack provides different functionality, and each device in the network uses all the parts of the stack to communicate with one another.

The physical layer is at the bottom and consists of all the hardware stuff, such as cables, switches, and connectors. This is where the fiber optic cables are. On top of the physical layer are the software layers that make up the protocol stack. In an FC SAN, those layers make up the Fibre Channel protocol. Every network uses a different protocol.

The Internet uses a protocol stack called the Transmission Control Protocol/Internet Protocol (TCP/IP). Both the Internet or a SAN can use fiber optic cables to transmit data. Fiber optic cables use light pulses to transmit data over the cable, which makes data move very fast. The only difference between regular fiber optic computer networks such as the Internet and a fiber optic storage area network such as a SAN is how the devices talk to each other over the network. SAN uses the Fibre Channel protocol, and the Internet uses the TCP/IP protocol.

How SAN makes computing different

Using a SAN can really change how you think about computing. In the past, you had the mainframe, which was a gigantic computer that could run all the programs in a large business. All the computer stuff was gathered in one place called a data center. All the storage that the mainframe needs was directly connected to it. Everything was located and managed as a single, large entity.

The PC revolution changed many things. Everything started to spread out. Data was moved off the mainframe and stored in server computers. The servers were then dispersed throughout the enterprise to bring computing power closer to the actual users. The servers became connected by a network, called a local area network, or LAN. This was cool because now the compute power was spread out and made more available to end users. Eventually, LANs were connected to create the Internet.

Networks enable people using computers in far-flung places to communicate and share information with each other. In business, problems arose when inter-networking finally took off. A great deal of data was now being stored with no effective way to manage it all. Everything was scattered everywhere, and managing all the data dispersed throughout the network became a nightmare.

Because all data storage was located inside each individual server, you had no effective way to efficiently allocate storage space between all the servers. Sure, users could share files over a LAN, but you still needed a way to share access to the physical disks in every server.

Hence the advent of the SAN.

Most high-performance applications need block-based access to store data on disk drives. The TCP/IP network protocol in a LAN does not natively provide a way to access the disk drives.

In a SAN, all the disk drives are stored in a dedicated storage device — a disk array. All the servers connect to the storage device over a high-speed network that uses the Fibre Channel protocol, which enables access to disks over a network. Using a SAN gives businesses the ability to consolidate access to data storage plus the ability to connect servers to that storage from anywhere on the network.

Putting a SAN in place makes the server computers less important. Servers become more peripheral to the data that's stored in the SAN. After all, the data is what is important to your business. If you lose a server, you can buy a new one. If you lose your data, it's "Adios, amigo," for your business.

Understanding the benefits of a SAN

The benefits of using a SAN are many because a SAN usually has a very high return on investment, makes the total cost of ownership of computing less, and has a pay-back period based in months instead of years.

Following is a list of just some of the ways you can expect a SAN to be beneficial.

  • Removes the distance limits of SCSI-connected disks: The maximum length of a SCSI bus is around 25 meters. Fibre Channel SANs allow you to connect your disks to your servers over much greater distances.
  • Greater performance: Fibre Channel SANs allow connection to disks at up to 200 megabytes per second today, with speeds of up to 1 gigabyte per second in the near future.
  • Increased disk utilization: SAN enables more than one server to access the same physical disk, which lets you allocate the free space on those disks more effectively.
  • Higher availability to storage by use of multiple access paths: A SAN allows for multiple physical connections to disks from a single or multiple servers.
  • Deferred disk procurement: Because you can use disk space more effectively, no space goes to waste, thus you don't need to buy disks as often as you used to.
  • Reduced data center rack/floor space: Because you don't need to buy big servers with room for lots of disks, you can buy fewer, smaller servers, which takes up less room.
  • New disaster recovery capabilities: This is a major benefit. SAN devices have the ability to mirror the data on the disks to another location. This can make your data safe if a disaster occurs.
  • Online recovery: By using online mirrors of your data in a SAN device, you can instantly recover your data if it becomes corrupt.
  • Better staff utilization: SANs enable fewer people to manage much more data.

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