SCSI - SCSI Command Protocol

SCSI Command Protocol

In addition to many different hardware implementations, the SCSI standards also include an extensive set of command definitions. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with iSCSI and serial SCSI. Other technologies which use the SCSI command set include the ATA Packet Interface, USB Mass Storage class and FireWire SBP-2.

In SCSI terminology, communication takes place between an initiator and a target. The initiator sends a command to the target, which then responds. SCSI commands are sent in a Command Descriptor Block (CDB). The CDB consists of a one byte operation code followed by five or more bytes containing command-specific parameters.

At the end of the command sequence, the target returns a status code byte, such as 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When the target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command in order to obtain a key code qualifier (KCQ) from the target. The Check Condition and Request Sense sequence involves a special SCSI protocol called a Contingent Allegiance Condition.

There are 4 categories of SCSI commands: N (non-data), W (writing data from initiator to target), R (reading data), and B (bidirectional). There are about 60 different SCSI commands in total, with the most commonly used being:

• 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.
• SCSI Read format capacities: Retrieve the data capacity of the device.
• 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.

Each device on the SCSI bus is assigned a unique SCSI identification number or ID. Devices may encompass multiple logical units, which are addressed by logical unit number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs.

A "direct access" (i.e. disk type) storage device consists of a number of logical blocks, addressed by Logical Block Address (LBA). A typical LBA equates to 512 bytes of storage. The usage of LBAs has evolved over time and so four different command variants are provided for reading and writing data. The Read(6) and Write(6) commands contain a 21-bit LBA address. The Read(10), Read(12), Read Long, Write(10), Write(12), and Write Long commands all contain a 32-bit LBA address plus various other parameter options.

The capacity of a "sequential access" (i.e. tape-type) device is not specified because it depends, amongst other things, on the length of the tape, which is not identified in a machine-readable way. Read and write operations on a sequential access device begin at the current tape position, not at a specific LBA. The block size on sequential access devices can either be fixed or variable, depending on the specific device. Tape devices such as half-inch 9-track tape, DDS (4 mm tapes physically similar to DAT), Exabyte, etc., support variable block sizes.