Just as you can never be too rich or too thin, you can never have too much storage capacity on your computer. At one time, a 5- or 10-Mbyte hard drive on a PC was a big deal. Today, 30- to 60-Gbyte drives are common on even the low-end PCs, with that capacity surging to many hundreds of gigabytes in network servers.
Large companies and government agencies routinely have storage capacities of many terabytes (Tbytes) or 1,000,000,000,000 (1012) bytes (or the binary equivalent 240 = 1,099,511,627,776). Storage-area networks (SANs) now make it possible to implement and manage the access to those huge storage facilities.
In fact, storage-area networking is the fastest growing computer networking technology today. A SAN is an interconnection of PCs, servers, and disk-drive subsystems that communicate over a fiber-optic network. SANs are used for massive storage of data and files, as well as system backup. The servers in the network communicate with the storage boxes via a special switch that allows any PC full, speedy access to any of many disk arrays. These SANs result from the huge need for extra storage capacity. Ballooning storage requirements can be traced to the Internet and e-mail, along with the need for backup, disaster recovery, and government regulations.
Many applications also require huge storage files to hold graphics such as video, digital photographs, and medical imaging. Research firm iSuppli (www.isuppli.com) estimates that use of Fibre Channel SAN ports has grown 30% annually for the past several years. The company also projects continued SAN growth at a compound annual growth rate (CAGR) of 31% through 2008. Let's look at the recent developments in SANs.
SAN TOPOLOGY AND ARCHITECTURES
Disk drives in servers are still primarily connected directly via the Small Computer Systems Interface (SCSI) bus in the smaller networks (Direct Attached Storage). However, two storage networking solutions have emerged over the years: Network Attached Storage (NAS) and SANs.
NAS systems use hard-disk arrays that are usually connected to the Ethernet LAN used by the server and PCs. They are given an IP address, and they access data in file format rather than block format, which is common for SANs. In this way, the files become accessible by virtually anyone on the local-area network (LAN) with authorization. NAS is a file-based (rather than a block-based) access method that uses file protocols like the Network File System (NFS) in UNIX/Linux systems and the Common Interface File System (CIFS) in Microsoft Windows systems.
For larger storage needs, a block-based SAN access system is preferred because it's easier to scale and provides access through any attached server. Figure 1 illustrates the generic SAN architecture. PCs and workstations are connected to the various servers by way of the enterprise LAN. Typically, the LAN is 100-Mbyte/s Fast Ethernet on CAT5 cables, although One Gigabit Ethernet (1GE) is becoming more common.
Then the servers are joined to the storage systems in either Redundant Arrays of Independent Disks (RAID) or Just a Bunch of Disks (JBOD) through switches. The transmission medium is almost universally fiber optics. In most SANs, the link is Fibre Channel (FC), the well known fiber-optic system designed specifically for block transfers of data between disk drives and servers. The SAN provides the most flexible method that lets any PC access anything, use tape backup, or store less frequently used files.
An alternative to FC interconnections is Ethernet connectivity using a newer protocol called Internet Small Computer Systems Interface (iSCSI). It encapsulates standard SCSI disk commands into TCP/IP packets for transmission over Ethernet or the Internet. While FC still dominates in most SANs, iSCSI systems are growing in number because the network connections usually cost less and in some cases use existing LAN connections rather than new runs of fiber. In a nutshell, iSCSI is making progress, but FC is keeping pace.
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