|Sood et al. Figure 1|
Second, in Sood et al's Table 6 their model predicts a mean time between steps of 1.25 years and a mean step size of 0.35 for magnetic disk. This leads them to predict an overall performance growth rate of 0.31 as against their version of Kryder's law at 0.28, not a large deviation compared to other technologies in their study.
|Sood et al. Figure 3|
Third, Sood et al's definition of performance for the desktop memory market is bytes per square inch (Table 2). Our definition of performance for long-term storage is bytes per dollar. My colleague Daniel Rosenthal has studied the relationship between these two metrics. It appears that about 3/4 of the decrease in $/GB can be attributed to the increase in bits/in2. But for the purpose of this discussion the important observation is that the price per byte of a new generation of disk technology typically starts out higher than that of its predecessor and decreases gradually until it drops from the market some considerable time after the introduction of its successor. On the shelves at Fry's we have:
- 4TB drives at $267
- 3TB drives at $140, $230, $130, $190, $150, $150
- 2TB drives at $190, $240, $120, $110, $110, $160, $130, $160, $105, $100, $180
- 1TB drives at $100, $80, $80, $80, $280, $80, $80, $100, $130, $80, $80, $70, $90
|Capacity (TB)||Cost ($/GB)|
Note that the latest technology (4TB) is still more expensive than its two most recent predecessors (3TB & 2TB). The previous technology (3TB) is now cheaper than its predecessors. This price evolution during the market life of each technology means that, despite Sood et al's conclusions, a continuous model would still be appropriate for our purposes even if the first two points are discounted.