In the Beginning, There Was SCSI…

Tuesday February 12, 2008 • Leave a Comment 

Any discussion of modern storage protocols, must begin with SCSI. From its humble beginnings in the 1980s, SCSI is nearly ubiquitous as the underlying technology used by most major storage protocols today. SCSI is an official standard architecture and is overseen by the T10 committee of the InterNational Committee for Information Technology Standards.

SCSI is both a physical cabling specification and a command set. For the purposes of this discussion, I will only be referring to SCSI’s logical definition. It’s been a long time since I’ve worried about SCSI cable lengths, wide, ultra-wide, HVD, LVD, etc. and I don’t plan to start now. Those days are gone.

The SCSI command protocol lives on however. At it’s core, SCSI is a client-server protocol with some special terminology. Here are some common SCSI concepts that are useful to know:

  • Initiator - The device which sends (initiates) the SCSI command. This is usually a disk adapter card in the computer system.
  • Target - The device which receives and processes the SCSI command. This is generally a storage device (i.e. disk or tape).
  • Bus - A physical or logical connection between a collection of initiators and targets. A bus typically contains a single initiator and many targets, but multi-initiator buses are allowed.
  • Logical Unit (LU) - The logical representation of a storage device. A logical unit may be a single SCSI hard drive, or as is common in large storage arrays, a logical unit may be a subdivision of the larger array. (It’s possible for a large storage array to contain hundreds of logical units.)
  • Logical Unit Number (LUN) -A numerical given to an individual logical unit that uniquely identifies it on a given bus. The term ‘LUN’ is commonly used to refer to the logical unit itself, but this usage is technically incorrect. LUNs are used to identify the source and destination for SCSI commands.

I’m not going to cover the specifics of the commands. For that, please visit the T10 site or Wikipedia. My intent for this installment was to provide these basic definitions and plant the idea that SCSI is the foundation for many of the protocols that we’ll be discussing in future installments.

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