SCSI: Difference between revisions

Revision as of 15:41, 1 August 2016

Revision as of 15:47, 13 August 2016

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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 ([[SCSI LBA|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 [[SCSI Read Commands|Read(6)]] and [[SCSI Write Commands|Write(6)]] commands contain a 21-bit LBA address. The [[SCSI Read Commands|Read(10)]], [[SCSI Read Commands|Read(12)]], [[SCSI Read Commands|Read Long]], [[SCSI Write Commands|Write(10)]], [[SCSI Write Commands|Write(12)]], and [[SCSI Write Commands|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 [[IBM 9 track|9-track tape]], [[Digital Data Storage|DDS]] (4 mm tapes physically similar to [[digital audio tape|DAT]]), [[Exabyte (company)|Exabyte]], etc., support variable block sizes.