Chapter6.SCSI Storage Fundamentals and SAN Adapters

Chapter 6. SCSI Storage Fundamentals and SAN Adapters

Upon completing this chapter, you will be able to

• Discuss the relevance of SCSI technology for storage networking

• Analyze SAN system and subsystem designs using the SCSI architecture model, including the roles of initiators and targets

• Explain the difference between a SCSI logical unit and a LUN and what both of them do

• Illustrate how tagged command queuing and extended copy functions work

• Describe the role HBAs have in SCSI processes in storage networks

In hindsight, it's relatively easy to analyze storage as having three distinct functions: filing, storing, and connecting. However, those distinctions were not always obvious due to the close integration of the storage and connecting functions. For example, Small Computer Systems Interface (SCSI) storage technology was initially developed as a combination of storing and connecting functions. When you bought SCSI products, you assumed they were going to connect to a parallel SCSI bus using standard SCSI connectors. Over time, there got to be many different bus and connector options to choose from, but the perception remained that the storage and the connection were the same intermingled technology. Today, with a number of bus and network connectivity options to chose from, it's much easier to see that SCSI storage processes and the underlying bus or network are distinct, independent technologies.

There is no way to understand the workings of network storage without knowing something about the architecture and logic operations of SCSI. SCSI processes are used in virtually all storage area network (SAN) implementations as the application layer that does storing work.

This chapter examines the logical architecture and structure of SCSI, including a discussion of the roles host bus adapters (HBAs) and storage controllers have in SCSI.

NOTE

Another protocol used in SANs is FICON (short for FIber CONnection). FICON is the newest version of IBM mainframe I/O technology designed to run on Fibre Channel (FC) networks. FICON's origins begin with the legacy mainframe Enterprise System Connection (ESCON) protocol that was introduced by IBM in 1991 as the first commercial implementation of a storage I/O network over fiber-optic cabling. ESCON works well but is limited by a transfer rate of 17 MBps and requires a relatively high number of physical connections and paths between systems and storage. FICON, as a protocol for FC networks, can take advantage of FC's greatly improved data transfer rates as well as its multiplexing capabilities. The result is faster performance along with much simpler, consolidated configurations.

Mainframe systems have a completely different storage I/O architecture than open-systems computers. The controller designs, transmission mechanisms, and data structures are all different from those used in SCSI. Therefore, FICON has a unique set of requirements and operating characteristics. As such, FICON is considered an advanced topic that is not covered in this book.