In computer data storage, a SCSI initiator is the endpoint that initiates a SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

 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 that does not initiate sessions, but instead waits for initiators' commands and provides required input/output

Input/output

In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...

 data transfers. The target usually provides to the initiators one or more LUNs, because otherwise no read or write command would be possible.

Detailed information

Typically, a computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 is an initiator and a data storage device

Data storage device

thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....

 is a target. As in a client–server architecture, an initiator is analogous to the client, and a target is analogous to the server. Each SCSI address (each identifier on a SCSI bus) displays behavior of initiator, target, or (rarely) both at the same time. There is nothing in the SCSI protocol that prevents an initiator from acting as a target or vice versa.

SCSI initiators are sometimes wrongly called SCSI controllers. See the SCSI host adapter

SCSI host adapter

A SCSI host adapter is a device used to connect one or more other SCSI devices to a computer bus. It is commonly called a SCSI controller, which is not strictly correct as any component understanding the SCSI protocol can be called a controller...

 article for a more detailed description of initiator's operation.

Other protocols

Initiator and target terms are applicable not only to traditional parallel SCSI

Parallel SCSI

Parallel SCSI is one of the interface implementations in the SCSI family. In addition to being a data bus, SPI is a parallel electrical bus: There is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it...

, but also to Fibre Channel Protocol

Fibre Channel

Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...

 (FCP), iSCSI

ISCSI

In computing, iSCSI , is an abbreviation of Internet Small Computer System Interface, an Internet Protocol -based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage...

 (see iSCSI target), HyperSCSI

HyperSCSI

HyperSCSI was a computer network protocol for accessing storage by sending and receiving SCSI commands.It was developed by researchers at the Data Storage Institute in Singapore in 2000 to 2003....

, (in some sense) SATA

Serial ATA

Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...

, InfiniBand

InfiniBand

InfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...

, DSSI

Digital Storage Systems Interconnect

The Digital Storage Systems Interconnect is a bus developed by Digital Equipment Corporation for connecting storage devices and clustering VAX systems and support was extended to MIPS based DECsystem and later to Alpha processor based AlphaServer systems.It was introduced in 1988 and has a...

 and many other storage networking protocols.

Address versus port

In most of these protocols, an address (whether it is initiator or target) is roughly equivalent to physical device's port. Situations where a single physical port hosts multiple addresses, or where a single address is accessible from one device's multiple ports are not very common, . Even when using multipath I/O

Multipath I/O

In computer storage, multipath I/O is a fault-tolerance and performance enhancement technique whereby there is more than one physical path between the CPU in a computer system and its mass storage devices through the buses, controllers, switches, and bridge devices connecting them.A simple example...

 to achieve fault tolerance

Fault-tolerant design

In engineering, fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level , rather than failing completely, when some part of the system fails...

, the device driver

Device driver

In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

 switches between different targets or initiators statically bound on physical ports, instead of sharing a static address between physical ports.

See also

• SCSI architectural model
  SCSI Architectural Model
  The SCSI architectural model provides an abstract view of the way that SCSI devices communicate. It is intended to show how the different SCSI standards are inter-related...
  
  
• iSCSI target