SAN vendors have been looking for alternatives to Fibre Channel that can combine the familiarity, bandwidth, and low cost of Ethernet with the efficient, speedy, block-based data transfer performance of a SAN. As you can imagine, lots of alternatives have come up to fill the gap, but the one that appears ready to take off is called SCSI over IP, or iSCSI.
Well go into more details of iSCSI below, and you can read our How iSCSI Works sidebar, but overall its a pretty simple concept--- its the mapping of SCSI commands (data and status) over a TCP/IP network. In an iSCSI based system, the user or application creates a command to store data on (or get data from) a SCSI-based storage system. The operating system processes the request and converts it to SCSI commands, which are then sent to software or an iSCSI card. The card or software encapsulates the data into a serial string of bytes preceded by an iSCSI header, and passes it to a TCP layer where it is packetized/encapsulated, and if necessary, encrypted for transfer over the network At the other end of the connection a storage controller decrypts the information in the packets, then uses the information in the iSCSI header to send the SCSI command/data to a SCSI drive, which performs the requested functions. If the request is for data retrieval, the data is encapsulated and returned.
The concept of iSCSI is simple, but very powerful--- you get all the benefits of block-based transactions and SCSI intelligence and compatibility, with the distance, bandwidth, standardization, low cost, and huge base of expertise available for Ethernet. What about all that TCP overhead? Theoretically you could use existing Ethernet cards with specialty iSCSI drivers to run SCSI over TCP/IP, but the TCP processing will eat CPU cycles for lunch and bog down performance. Instead, a whole new crop of cards that perform TCP protocol processing and sometimes iSCSI processing on board through special ASICs will rev up performance to Fibre Channel levels and beyond.