SCSI initiator and target
Endpoints that initiate or respond to SCSI commands over a network.
In computer data storage, a SCSI initiator is the endpoint that initiates a SCSI session by sending a SCSI command over a network, while a SCSI target is the endpoint that waits for commands and provides input/output data transfers. The initiator usually does not provide Logical Unit Numbers (LUNs), whereas the target typically provides one or more LUNs to enable read or write commands.
- Analogous architecture
- Client–server, where initiator is client and target is server
- Common roles
- Computer is initiator, data storage device is target
- Protocols using terms
- Parallel SCSI, Fibre Channel Protocol (FCP), iSCSI, HyperSCSI, SATA, ATA over Ethernet (AoE), InfiniBand, DSSI
Lore & Background
The SCSI initiator and target roles are defined by behavior on a SCSI bus, where each SCSI address displays behavior as initiator, target, or rarely both simultaneously. 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 controllers.
These terms apply not only to traditional parallel SCSI but also to Fibre Channel Protocol (FCP), iSCSI, HyperSCSI, SATA, ATA over Ethernet (AoE), InfiniBand, DSSI, and many other storage networking protocols. In most of these protocols, an address (whether initiator or target) is roughly equivalent to a physical device's port. Situations where a single physical port hosts multiple addresses, or where a single address is accessible from multiple ports of one device, are not very common. Even when using multipath I/O for fault tolerance, the device driver switches between different targets or initiators statically bound on physical ports, rather than sharing a static address between ports.
Reader's Guide
The distinction between SCSI initiator and target is fundamental to storage networking, establishing a clear client–server model where the initiator (typically a computer) issues commands and the target (typically a storage device) responds with data transfers. This architecture enables standardized communication across diverse protocols such as Fibre Channel, iSCSI, and InfiniBand, allowing storage systems to interoperate regardless of underlying transport. The terms persist even in modern protocols like ATA over Ethernet and SATA, demonstrating their enduring relevance. Notably, the SCSI protocol does not enforce a strict role; an initiator could theoretically act as a target, though this is rare in practice. The concept of addresses mapping roughly to physical ports, with limited support for multiple addresses per port or shared addresses across ports, shapes how multipath I/O and fault tolerance are implemented—by switching between statically bound targets or initiators rather than sharing addresses. This framework has influenced storage networking design for decades, providing a consistent vocabulary for describing endpoint roles in data access.
Frequently Asked Questions
Who is SCSI initiator and target?
In the world of computer storage, the SCSI initiator is the endpoint that kicks off a session by firing off a command across the network, while the SCSI target is the endpoint that patiently waits for those commands and handles the actual data transfer. Think of it as a client–server pairing where the initiator plays the client role and the target plays the server role.
What are SCSI initiator and target's powers/role?
The initiator's job is to generate and send SCSI commands to request actions, while the target's job is to receive those commands and supply or accept input/output data. The target typically exposes one or more Logical Unit Numbers (LUNs) so the initiator knows where to direct read or write operations, whereas the initiator generally does not provide LUNs itself.
Why is SCSI initiator and target important?
This initiator–target model gives networked storage a clean, role-based structure that keeps command flow unambiguous and scalable. It underpins a wide range of protocols—including Parallel SCSI, Fibre Channel, iSCSI, SATA, and AoE—making it a foundational pattern in how modern systems talk to their disks.
Who are SCSI initiator and target's allies (protocols)?
The initiator and target duo appears across many transport protocols, from classic Parallel SCSI and Fibre Channel Protocol to iSCSI, HyperSCSI, SATA, ATA over Ethernet, InfiniBand, and DSSI. Each protocol adapts the same core initiator-commands / target-responds pattern to its own physical or network layer.
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