Английская Википедия:Five-minute rule
Шаблон:Distinguish In computer science, the five-minute rule is a rule of thumb for deciding whether a data item should be kept in memory, or stored on disk and read back into memory when required. It was first formulated by Jim Gray and Gianfranco Putzolu in 1985,[1][2] and then subsequently revised in 1997[3] and 2007[4] to reflect changes in the relative cost and performance of memory and persistent storage.
The rule is as follows:
The 5-minute random rule: cache randomly accessed disk pages that are re-used every 5 minutes or less.
Gray also issued a counterpart one-minute rule for sequential access:[5]
The 1-minute rule: cache sequentially accessed disk pages that are re-used every 1 minute or less.
Although the 5-minute rule was invented in the realm of databases, it has also been applied elsewhere, for example, in Network File System cache capacity planning.[6]
The original 5-minute rule was derived from the following cost-benefit computation:[4]
- BreakEvenIntervalinSeconds = (PagesPerMBofRAM / AccessesPerSecondPerDisk) × (PricePerDiskDrive / PricePerMBofRAM)
Applying it to 2007 data yields approximately a 90-minutes interval for magnetic-disk-to-DRAM caching, 15 minutes for SSD-to-DRAM caching and 2Шаблон:Frac hours for disk-to-SSD caching. The disk-to-DRAM interval was thus a bit short of what Gray and Putzolu anticipated in 1987 as the "five-hour rule" was going to be in 2007 for RAM and disks.[4]
According to calculations by NetApp engineer David Dale as reported in The Register, the figures for disc-to-DRAM caching in 2008 were as follows: "The 50KB page break-even was five minutes, the 4KB one was one hour and the 1KB one was five hours. There needed to be a 50-fold increase in page size to cache for break-even at five minutes." Regarding disk-to-SSD caching in 2010, the same source reported that "A 250KB page break even with SLC was five minutes, but five hours with a 4KB page size. It was five minutes with a 625KB page size with MLC flash and 13 hours with a 4KB MLC page size."[7]
In 2000, Gray and Shenoy applied a similar calculation for web page caching and concluded that a browser should "cache web pages if there is any chance they will be re-referenced within their lifetime."[8]
References
- ↑ Шаблон:Citation
- ↑ Шаблон:Citation
- ↑ Шаблон:Citation
- ↑ 4,0 4,1 4,2 Шаблон:Citation Free version in ACM Queue, September 2008.
- ↑ Шаблон:Cite book
- ↑ Шаблон:Cite book
- ↑ Шаблон:Cite web
- ↑ Jim Gray, Prashant Shenoy, "Rules of Thumb in Data Engineering", MS-TR-99-100
