Spin Speed
The rotational speed of the disk, commonly referred to as ‘rpm’, has changed in recent years from 7200 to 10,000, and now to 15,000 rpms. Increased spin speed results in reduced mechanical latency, which makes for a higher performing drive. A faster spin speed can also result in a drive that consumes more power and dissipates more heat. However, by reducing the rotating mass of the drive, these effects can be negated. Our drives are selected upon measures of the power consumption. Only drives with equal or lower power consumption characteristics relative to the original are allowed to be substituted into older applications.

Interface
Since 1981, there have been several generations of the SCSI protocol. Each new generation has doubled the performance of the previous one. SCSI performance has ranged from an 8-bit, single-ended (SE) interface transferring data up to 4 MB/s to the latest 16-bit, low-voltage differential (LVD) interface transferring data at 320 MB/s per channel. The drive communicates with the host controller across this interface, and much work has been done throughout the industry to retain backwards compatibility as new interfaces have been introduced. Although there have been many generations of SCSI, there are only three basic SCSI specifications: SCSI-1, SCSI-2, and SCSI-3. Table 1 lists the many variations of these specifications.

Specification		SCSI Name	Bus Width	Bus Speed	Data Rate	Signaling	Method
						SE	LVD
SCSI-1		Async SCSI	8 bits	5 MHz	4 MB/s	X
SCSI-1		Sync SCSI	8 bits	5 MHz	5 MB/s	X
SCSI-2		Fast SCSI	8 bits	10 MHz	10 MB/s	X
SCSI-2		Fast/Wide SCSI	16 bits	10 MHz	20 MB/s	X
SCSI-3		Wide Ultra SCSI	16 bits	10 MHz	40 MB/s	X
SCSI-3		Wide Ultra2 SCSI	16 bits	10 MHz	80 MB/s		X
SCSI-3		Ultra160 SCSI	16 bits	10 MHz	160 MB/s		X
SCSI-3		Ultra320	16 bits	10 MHz	320 MB/s		X

Drives utilizing the newer interfaces can be substituted into older applications as long as the physical connections are compatible. The interface speed at which the drive and host controller operate at will be the slower of the two devices. Each drive on the bus negotiates independently with the host controller.

Capacity
Capacity is the disk drive parameter that has experienced the most growth. Steady increases in areal density have pushed the size of SCSI drives from 4.3-Gbyte to 181-Gbyte in less than ten years, and the new technology to be release in the 4th quarter 2004 boasting 300- and 147-Gbyte in the seventh generation of 10K-RPM products. The amount of data that can be put onto a single platter dictates the minimum capacity point for a given generation of product. Cuurently that is 36GB, but soon that capacity will increase to 73-Gbyte. Typically, substituting a higher capacity drive for a smaller capacity has no effect on the performance of an application. When a larger drive is introduced into an existing RAID set, the additional capacity above that of the existing set is unusable. However, this has no effect on the operation. Smaller capacity drives can be artificially created by limiting the useable capacity of a drive through the firmware. Essentially, the firmware only allows a given amount of the total capacity to be accessed. RAID controllers employs this method in order to provide lower capacity drives after the native capacity units are no longer available. The e4 qualification process includes testing of these drives to ensure continued compatibility in customer systems.