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SCSI Interfaces Guide


Protocol vs bus

Once upon a time, the Small Computer Systems Interface defined both a parallel I/O bus and a data protocol to connect a wide variety of peripherals (disk drives, tape drives, modems, printers, scanners, optical drives, test equipment, and medical devices) to a host computer.

Although the old parallel (fast/wide/ultra) SCSI bus has largely fallen out of use, the SCSI command set is more widely used than ever to communicate with devices over a number of different busses.

SCSI protocolThe SCSI protocol is a big-endian peer-to-peer packet based protocol. SCSI commands are 6, 10, 12, or 16 bytes long, often followed by an associated data payload.

I20iSCSIParallel portsSCSI commands can be transported over just about any kind of bus, and are the default protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, and ATAPI devices. SCSI packets are also commonly exchanged over Infiniband, I20, TCP/IP (iSCSI), even Parallel ports.

Design of the Linux SCSI subsystem

The SCSI subsystem uses a three layer design, with upper, mid, and low layers. Every operation involving the SCSI subsystem (such as reading a sector from a disk) uses one driver at each of the 3 levels: one upper layer driver, one lower layer driver, and the SCSI midlayer.

The SCSI upper layer provides the interface between userspace and the kernel, in the form of block and char device nodes for I/O and ioctl(). The SCSI lower layer contains drivers for specific hardware devices.

In between is the SCSI mid-layer, analogous to a network routing layer such as the IPv4 stack. The SCSI mid-layer routes a packet based data protocol between the upper layer's /dev nodes and the corresponding devices in the lower layer. It manages command queues, provides error handling and power management functions, and responds to ioctl() requests.

SCSI upper layer

The upper layer supports the user-kernel interface by providing device nodes.

sd (SCSI Disk)

sd (sd_mod.o)


sr (sr_mod.o)

st (SCSI Tape)

st (st.o)

sg (SCSI Generic)

sg (sg.o)

ch (SCSI Media Changer)

ch (ch.c)

SCSI mid layer

SCSI midlayer implementation

Transport classes

Transport classes are service libraries for drivers in the SCSI lower layer, which expose transport attributes in sysfs.

SCSI lower layer

Host Bus Adapter transport types

Many modern device controllers use the SCSI command set as a protocol to communicate with their devices through many different types of physical connections.

In SCSI language a bus capable of carrying SCSI commands is called a “transport”, and a controller connecting to such a bus is called a “host bus adapter” (HBA).

About This Book


  • James Bottomley
  • Rob Landley

Copyright 2007

  • Linux Foundation

Legal Notice

This documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. For more details see the file COPYING in the source distribution of Linux.