Endnotes

1. The Tape Automation Coalition (TAC) was being formed by Hewlett Packard, ADIC, Quantum Corporation and a number of other vendors at the time of this writing.

2. Jon William Toigo, "Avoiding the Data Crunch," Scientific American , May 2000.

3. Not to minimize IBM's accomplishment in any way, the company discovered a way to push back the data storage industry's most formidable barrier ”the areal density limit ”by adding a few atoms of "pixie dust" to its platter coating process. Following on a long list of accomplishments in hard disk development, IBM was first to mass-produce computer hard disk drives using the new type of magnetic coating, and effectively quadrupled hard disk data densities . Known technically as "anti-ferromagnetically- coupled (AFC) media," the new multi-layer coating used a three-atom-thick layer of the element ruthenium, a precious metal similar to platinum , sandwiched between two magnetic layers . That only a few atoms could have such a dramatic impact caused some IBM scientists to refer to the ruthenium layer informally as "pixie dust." AFC media is now shipping in IBM's Travelstar notebook hard disk drive products with data densities of up to 25.7 gigabits per square inch. In time, IBM plans to implement AFC media across all of its disk drive product lines and hopes to reach areal densities of 150 Gb/in. ^[] within the next two years .

4. Marty Czekalski, Vice President, Director ”SCSI Trade Association, Technical Marketing Manager ”Maxtor Corporation, "Serial Attached SCSI," March 5, 2003, www.serialattachedscsi.com .

5. Serial ATA Working Group, "Serial ATA White Paper," November 7, 2000, www.serialata.org .

6. Serial ATA Working Group, "Serial ATA (SATA) in Servers and Networked Storage," June 7, 2002, www.serialata.org .

7. Harry Mason, Director, Industry Marketing, LSI Logic and President, SCSI Trade Association, "Serial Attached SCSI: The Universal Enterprise Storage Connection," March 5, 2003, www.serialattachedscsi.com .

8. Suggested by SCSI Trade Association Presentation by Seagate Technology, "It's More than the Interface: Selecting the Right Drive for Your Application," August 20, 2002, www.serialattachedscsi.com .

9. See Serial ATA Working Group, "Serial ATA (SATA) in Servers and Networked Storage," June 7, 2002, www.serialata.org for more models of Serial ATA topologies and arrays.

10. SCSI Trade Association Presentation by Seagate Technology, "It's More than the Interface: Selecting the Right Drive for Your Application," August 20, 2002, www.serialattachedscsi.com .

    An additional insight about the value of MTBF as a criterion for disk drive selection is offered by John R. Vacca, himself an author and IT Consultant with Tech Write in Pomeroy, Ohio, and a technical reviewer for this book. Says Vacca, "MTBF is an excellent characteristic for determining how many spare hard drives are needed to support 1000 PC's, but a poor characteristic for guiding you on when you should change your hard drive to avoid a crash. MTBF's are best determined from large populations. How large? From every point of view (theoretical, practical, statistical) but cost, the answer is 'the larger, the better.' There are well established techniques for planning and conducting test programs to develop specified levels of confidence in a hard drive's MTBF. Establishing an MTBF at the 80% confidence level, for example, is clearly better, but much more difficult and expensive, than doing it at a 60% confidence level. As an example, a test designed to demonstrate a hard drive's MTBF at the 80% confidence level, requires a total hard drive-time of 160% of the MTBF if it can be conducted with no failures. You don't want to know how much hard drive-time is required to achieve reasonable confidence levels if any failures occur during the test."

    "What, by the way is, 'hard drive-time?' An important subtlety is that 'hard drive-time' isn't 'clock time' (unless, of course, your hard drive is a clock). The question of how to compute hard drive-time is a critical one in reliability engineering. For some hard drives (living thing) time always counts, but for others the passage of hard drive-time may be highly dependent upon the state of the hard drive. Various ad hoc time corrections (such as "power on hours" (POH)) have been used, primarily in the electronics area. There is significant evidence that, in the mechanical area, hard drive-time is much more related to activity rate than it is to clock time. Measures such as 'Mean Cycles Between Failures (MCBF)' are becoming accepted as more accurate ways to assess the 'duty cycle effect.' Well-founded, if heuristic, techniques have been developed for combining MCBF and MTBF effects for systems in which the average activity rate is known."

    "MTBF need not then be 'Mysterious time Between Failures' or 'Misleading Time Between Failures,' but an important system characteristic which can help to quantify the suitability of a system for a potential application. While rising demands on system integrity may make this characteristic seem ' unnatural ', remember you live in a country of 330 million 10-million- hour MTBF people!"

11. Figure 6-12 is very simplified. There are, of course, new disk drives appearing in the market daily that span categories shown here. For example, some vendor will doubtless provide a high performance disk with high capacity using the SAS interconnect and may also offer a high performance/low capacity SAS drive to support other applications such as notebook computing. So, these categories are by no means "cut and dried ." Planners will need to read all specifications carefully .

12. Jon William Toigo, "SCSI versus Fibre Channel: Picking The Right Protocol," Enterprise Storage Strategies Newsletter, June 5, 2003, www.esj.com .