Introduction to Storage Area Networks

Storage Area Networking

A storage area network (SAN) is any high-performance network whose primary purpose is to enable storage devices to communicate with computer systems and with each other. We can also define the SAN network, as the network behind the servers. 

Even though this difference in protocols might seem trivial, it actually hints at the very essence of a SAN. Networks that depend on TCP/IP and SMB are primarily designed to access file system data. In other words, these types of networks are ideally suited for reading and writing files that are stored on file servers, Web servers, etc.
In contrast, Fibre Channel doesn’t work at the file level, but rather at the storage block level. As such, you wouldn’t use Fibre Channel to read a file that is stored on a file share. Instead, Fibre Channel reads and writes individual storage blocks.

A SAN can be used to bypass traditional network bottlenecks. It facilitates direct, high-speed data transfers between servers and storage devices, potentially in any of the following three ways:

1. Server to storage: This is the traditional model of interaction with storage devices. The advantage is that the same storage device might be accessed serially or concurrently by multiple servers.
2.   Server to server: A SAN might be used for high-speed, high-volume communications between servers.

3. Storage to storage: This outboard data movement capability enables data to be moved without server intervention, therefore freeing up server processor cycles for other activities like application processing. Examples include a disk device that backs up its data to a tape device without server intervention, or a remote device mirroring across the SAN.

Here is a small block diagram of SAN components that are building blocks of the SAN Fabric.

Fibre Channel connectivity is a bit more interesting. Hosts connect to a Fibre Channel SAN by using a host bus adapter. A host bus adapter is similar to a network card in that it gets installed into a slot within the server and provides connectivity to the SAN. The primary difference between host bus adapters and Ethernet adapters is that host bus adapters are designed to work with fiber optic cables rather than Ethernet cables. Another difference is that unlike Ethernet, Fibre Channel does not use the TCP/IP protocol or carry Ethernet packets.

Although network throughput does play a role in the overall speed of the connection, the main reason why Fibre Channel is so much faster than TCP/IP is because Fibre Channel is a more efficient protocol with less overhead. Having less overhead allows Fibre Channel to move data more quickly.

Here we can take a look of Cisco diagram that will show us the simplicity of the SAN frame.

 

The Fibre Channel Protocol (FCP) is the interface protocol of SCSI on Fibre Channel (FC). It is a gigabit speed network technology that is primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee of Information Technology Standards (INCITS), an ANSI accredited standards committee. It started for use primarily in the supercomputer field, but is now the standard connection type for SANs in enterprise storage. Despite its name, Fibre Channel signaling can run on both twisted-pair copper wire and fiber optic cables.

Fibre Channel Protocol (FCP) is a transport protocol (similar to TCP used in IP networks) that predominantly transports SCSI commands over Fibre Channel networks.

Fibre Channel Variants

NAME	Line-rate (GBaud)	Throughput (full duplex; MB/s)*	Availability
1GFC	1.0625	200	1997
2GFC	2.125	400	2001
4GFC	4.25	800	2004
8GFC	8.5	1,600	2005
10GFC	10.52	2,550	2008
16GFC	14.025	3,200	2011
32GFC	28.05	6,400	2014 (announced)

Switched fabric is probably the most widely used Fibre Channel topology today. It is by far the most flexible of the three topologies, but it is also the most expensive to implement. When it comes to SANs however, you usually get what you pay for.

I hope that I have shed some light on how SANs work. There is more to come.

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