{{No footnotes, date=May 2025
The SCSI architectural model provides an abstract view of the way that
SCSI
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices, best known for its use with storage devices such as hard disk drives. SCSI was introduced ...
devices communicate. It is intended to show how the different SCSI standards are inter-related. The main concepts and terminology of the SCSI architectural model are:
* Only the externally observable behavior is defined in SCSI standards.
* The relationship between SCSI devices is described by a client-server service-delivery model. The client is called a
SCSI initiator
In computer data storage, a SCSI initiator is the endpoint that initiates a SCSI session, that is, sends a SCSI command. The initiator usually does not provide any Logical Unit Numbers (LUNs).
On the other hand, a SCSI target is the endpoint tha ...
and the server is called a SCSI target.
* A SCSI domain consists of at least one SCSI device, at least one SCSI target and at least one SCSI initiator interconnected by a service delivery subsystem.
* A SCSI device has one or more SCSI ports, and a SCSI port may have an optional SCSI port identifier (SCSI ID or PID).
* A SCSI device can have an optional SCSI device name which must be unique within the SCSI domain in which the SCSI device has SCSI ports. This is often called a
World Wide Name
A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier used in storage technologies including Fibre Channel, Parallel ATA, Serial ATA, SCSI and Serial Attached SCSI (SAS).
A WWN may be employed in a variety of roles, s ...
. Note that the "world" may only consist of a very small number of SCSI devices.
* A SCSI target consists of one or more logical units (LUNs), which are identified by logical unit numbers.
* A LUN may have dependent LUNs embedded within it. This can recur up to a maximum nesting depth of four addressable levels.
* There are three type of SCSI ports: initiator ports, target ports and target/initiator ports. A SCSI device may contain any combination of initiator ports, target ports and target/initiator ports.
* SCSI distributed objects are considered to communicate in a three layer model:
** The highest level of abstraction is the SCSI Application Layer (SAL) where an initiator and a target are considered to communicate using
SCSI command
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices, best known for its use with storage devices such as hard disk drives. SCSI was introduced ...
s sent via the SCSI application protocol.
** The SCSI Transport Protocol Layer (STPL) is where an initiator and a target are considered to communicate using a SCSI transport protocol. Examples of SCSI transport protocols are
Fibre Channel
Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to Server (computing), servers in storage area networks (SAN) in ...
iSCSI
Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...
and the SCSI Parallel Interface.
** The lowest level is the SCSI Interconnect Layer (SIL) where an initiator and a target are considered to communicate using an interconnect. It consists of the services, signaling mechanism and interconnect subsystem used for the physical transfer of data from an initiator to a target.
* A SCSI task is represented by an I_T_L_Q nexus. This is where one Initiator Port talks to one Target Port, addressing one LUN and together they execute one task (identified by Q).
Architectural Model
An architectural model is a type of scale model made to study aspects of an architectural design or to communicate design intent. They are made using a variety of materials including paper, plaster, plastic, resin, wood, glass, and metal.
Mod ...