SCSI

SCSI (hereinafter, the 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, electrical and optical interfaces. SCSI is most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

Curriculum

Main wikipage: CNM Cloud requirements

Cabling

SCSI Parallel Interface

Internal parallel SCSI cables are usually ribbons, with two or more 50–, 68–, or 80–pin connectors attached. External cables are typically shielded (but may not be), with 50– or 68–pin connectors at each end, depending upon the specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices.

Fibre Channel

Fibre Channel can be used to transport SCSI information units, as defined by the Fibre Channel Protocol for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber.

Serial Attached SCSI

uses a modified Serial ATA data and power cable.

iSCSI

iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP.

SRP

The SCSI RDMA Protocol (SRP) is a protocol that specifies how to transport SCSI commands over a reliable RDMA connection. This protocol can run over any RDMA-capable physical transport, e.g. InfiniBand or Ethernet when using RoCE or iWARP.

USB Attached SCSI

USB Attached SCSI allows SCSI devices to use the Universal Serial Bus.

Automation/Drive Interface

The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with the controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of RS-422 for the physical connections. The second-generation ADT-2 standard defines iADT, use of the ADT protocol over IP (Internet Protocol) connections, such as over Ethernet. The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations.

SCSI Command Protocol

• Test unit ready: Queries device to see if it is ready for data transfers (disk spun up, media loaded, etc.).
• Inquiry: Returns basic device information.
• Request sense: Returns any error codes from the previous command that returned an error status.
• Send diagnostic and Receive diagnostic results: runs a simple self-test, or a specialised test defined in a diagnostic page.
• Start/Stop unit: Spins disks up and down, or loads/unloads media (CD, tape, etc.).
• Read capacity: Returns storage capacity.
• Format unit: Prepares a storage medium for use. In a disk, a low level format will occur. Some tape drives will erase the tape in response to this command.
• Read (four variants): Reads data from a device.
• Write (four variants): Writes data to a device.
• Log sense: Returns current information from log pages.
• Mode sense: Returns current device parameters from mode pages.
• Mode select: Sets device parameters in a mode page.

Device type

While all SCSI controllers can work with read/write storage devices, i.e. disk and tape, some will not work with some other device types; older controllers are likely to be more limited, sometimes by their driver software, and more Device Types were added as SCSI evolved. Even CD-ROMs are not handled by all controllers. Device Type is a 5-bit field reported by a SCSI Inquiry Command; defined SCSI Peripheral Device Types include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed.

SCSI Enclosure Services

In larger SCSI servers, the disk-drive devices are housed in an intelligent enclosure that supports SCSI Enclosure Services (SES). The initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics.