The design of data management systems has always been influenced by the storage hardware landscape. In the 1980s, database engines used a two-tier storage hierarchy consisting of dynamic random access memory (DRAM) and hard disk drives (HDD). Given the disparity in cost between HDD and DRAM, it was important to determine when it made economic sense to cache data in DRAM as opposed to leaving it on the HDD.
In 1987, Jim Gray and Gianfranco Putzolu established the five-minute rule that gave a precise answer to this question: "1KB records referenced every five minutes should be memory resident."9 They arrived at this value by using the then-current price-performance characteristics of DRAM and HDD shown in Table 1 for computing the break-even interval at which the cost of holding 1KB of data in DRAM matches the cost of I/O to fetch it from HDD.
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