Manage servers and SANs better by understanding SCSI

Most servers use SCSI drives, which typically perform better than the IDE hard drives prevalent in workstations and consumer-grade computers. SANs pretty much use SCSI exclusively. In short, it's almost impossible

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for an administrator to allocate storage in a SAN, build a cluster or set up a hardware-based RAID array without understanding how SCSI works.

In this article and others to follow, I'll distinguish between IDE and SCSI, explain how a SCSI subsystem works and differentiate between the various flavors of SCSI.

SCSI drives are similar to IDE drives, but the architectural differences between them center around the SCSI bus. On a system that uses IDE drives, the IDE controller is considered part of the system bus. The IDE controller communicates directly with the hard disk controller, which is built into the hard drive. In short, an IDE hard drive communicates directly with a computer's system bus.

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However, SCSI drives cannot communicate directly with the system bus. They communicate with the system, and with each other, through the SCSI bus. The system bus communicates with the SCSI bus through the SCSI host adapter. The SCSI host adapter is usually a PCI (or PCI Express) expansion card that contains the hardware needed to attach SCSI drives. (Some servers have a SCSI host adapter integrated into the system board.)

Most SCSI host adapters contain a set of pins that a ribbon cable can be connected to. This allows you to connect internal drives to the SCSI host adapter the same way you'd connect an IDE drive to an IDE controller. Most SCSI host adapters also contain a port you can use to connect external drives or other SCSI devices. SCSI devices are usually drives, but other devices, such as scanners, are designed to be connected to the SCSI bus.

Note: The actual implementation of a SCSI host adapter varies by manufacturer. But in most cases, if a SCSI host adapter supports internal and external drives, both kinds of drives can be connected to the SCSI host adapter simultaneously, and all these drives operate as a part of the same SCSI bus.

SCSI hard drives: How they differ from IDE drives

SCSI hard drives are not that different from IDE hard drives. IDE hard drives are designed so that the disk controller can communicate directly with the system bus. This is not an option for SCSI drives, so SCSI hard drives contain a SCSI interface controller chip that allows the drive to work as a part of the SCSI bus. In fact, some SCSI hard drives are completely identical to their IDE counterparts, aside from this one extra chip and the different connector for the data ribbon.

SCSI drives tend to be significantly more expensive than comparable IDE drives – not because of the added SCSI interface controller chip, but because they're intended for use in servers. They feature lower seek times and higher RPMs than their IDE counterparts, and are usually of higher quality.

The diagram below shows the architectural differences between IDE and SCSI. The left side of the diagram shows how an IDE drive is able to communicate directly with a computer's system bus. (Remember, the hard disk controller is built into the drive). The right side of the diagram shows that a computer using a SCSI drive has all the same components as a system using IDE drives. The difference is that a SCSI hard drive contains a SCSI interface controller chip that allows it to communicate across the SCSI bus, which is implemented by the SCSI host adapter.

That the system bus can't communicate directly with a SCSI drive raises this question: How is it possible to boot from a SCSI hard drive? Answer: Most systems have a mini-SCSI driver built into their BIOS. This allows the system to access a SCSI drive prior to an operating system being loaded, so the system can boot from the SCSI drive.

Note: In most cases, if a system contains an IDE drive and a SCSI drive, the IDE drive will be used as the boot device. A few systems have options to boot from a SCSI drive even if an IDE drive is present, but not many.

My next article will discuss how multiple devices can be connected to a SCSI bus.

About the author: 
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server, Exchange Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. He writes regularly for SearchWinComputing.com and other TechTarget sites.

This was first published in April 2007

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