What are the reuse time thresholds for these three technology generations?

Keeping "frequently used" (or "hot") pages in DRAM can save disk accesses, but how do we determine the exact meaning of "frequently used" for a given system? Data engineers use the cost ratio between DRAM and disk access to quantify the reuse time threshold for hot pages. The cost of a disk access is $Disk /accesses_per_ sec, while the cost to keep a page in DRAM is $DRAM_MB/page_size. The typical DRAM and disk costs and typical database page sizes at several time points are listed below:

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Year 1987 1997 2007 DRAM Cost ($/MB) 5000 15 0.05 Page Size (KB) 1 8 64 Disk Cost ($/disk) 15000 2000 80 Disk Access Rate (access/sec) 15 64 83