One of the earliest truisms in computing is "Data will grow to fill all available space." We used to address this problem by building bigger and bigger servers with more disk drives. Ultimately this approach resulted in some servers running out of disk space and slots for additional drives, while others had plenty of free disk space. The storage area network was invented to allow multiple servers to share space on common larger, expandable disk arrays, thereby improving disk utilization and simplifying the process of allocating disk space to servers. In addition to just sharing a disk array, storage area networks can also provide data replication, snapshot and high-speed backup facilities.
Until recently storage area networks were built using Fibre Channel, a 1-4Gbps network designed specifically for SAN applications. Fibre Channel is well suited to organizations with mainframe or other servers that require very high storage performance.
Organizations primarily using Windows or Linux on Intel servers can get all the benefits of a storage area network using the iSCSI protocol that uses TCP/IP and gigabit or 10Gb Ethernet to run a SAN. Since these organizations have staff, or VARs, that know TCP/IP and Ethernet, they can adopt iSCSI easily while spending half or less than a Fibre Channel system would cost.
Customers that need more than one external drive array for their servers should have a storage area network. Current iSCSI arrays with basic RAID features aren't significantly more expensive than external SCSI arrays.
Storage area networks are also a natural fit for customers implementing server virtualization. With a virtual server hosted on a storage area network array, you can move a virtual server from one host to another by just reconnecting the logical drive that houses that virtual server to the new host.
Dig Deeper on Primary and secondary storage
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