By Leon Erlanger  |  Posted 2012-04-30 Print this article Print

: How iSCSI Works">

As its name implies, iSCSI is based on the existing SCSI (Small Computer Systems Interface) standards currently used for communication among servers and their attached storage devices. SCSI has two types of devices-- SCSI initiators (usually servers, which start the communications by issuing the commands to be executed) and SCSI Targets (usually storage devices, which respond to the initiators and carry out the commands). Targets consist of a number of "logical units" (LUs) that are directly addressable and execute the SCSI commands. Commands are communicated via a structure of "Command Descriptor Blocks" (CDBs) and are often combined into "tasks."
SCSI Command Descriptor Block (CDB) Format
Bit 7 6 5 4 3 2 1 0








0 Operation Code


Command Specific Parameters 




n Control

Courtesy of Cisco Sytems, Inc.
Commands are executed with data phases, in which the data travels from the initiator to the target (typical of a WRITE command), or from the target to the initiator (as in a READ command), and with status phases, in which the target completes the operation and terminates the SCSI command or task. iSCSI gives SCSI initiators and targets unique URL-like names, and provides a method for their discovery and mutual authentication. These names can be assigned by OS vendors, NIC and driver vendors, device vendors, gateway vendors, or even service providers and customers.


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