practice

Doi: 10.1145/1538788.1538805

was about equal to the (fractional) price of a disk drive required to access such a record every 400 seconds, which they rounded to five minutes. The break-even interval is about inversely proportional to the record size. Gray and Putzolu reported one hour for 100-byte records and two minutes for 4KB pages.

BY Goe Tz GRAefe

The five-minute rule was reviewed and renewed 10 years later. ¹⁴ Lots of prices and performance parameters had changed (for example, the price of RAM had tumbled from $5,000 to $15 per megabyte). Nonetheless, the break- Minute Rule even interval for 4KB pages was still around five minutes. The first goal of this article is to review the five-minute rule after another 10 years.

20 Years Later

Of course, both previous articles acknowledged that prices and performance vary among technologies and devices at any point in time (and how flash Memory (RAM for mainframes versus minicomputers, SCSI versus IDE disks, and so on). Interested readers are invited to Changes the Rules) reevaluate the appropriate formulas for their environments and equipment. The values used here (in Table 1) are meant to be typical for 2007 technologies rather than universally accurate.

In addition to quantitative changes in prices and performance, qualitative changes already under way will affect the software and hardware architectures of servers and, in particular, database systems. Database software will change radically with the advent of new technologies: virtualization with hardware and software support, as well as higher utilization goals for physical machines; many-core processors and transactional memory supported both in programming environments and hardware; ²⁰ deployment in containers housing thousands of processors and many terabytes of data; ¹⁷ and flash memory that fills the gap between traditional RAM and traditional rotating disks.

Flash memory falls between traditional RAM and persistent mass storage based on rotating disks in terms of acquisition cost, access

Article development led by queue.acm.org

Revisiting Gray and Putzolu’s
famous rule in the age of Flash.
The five-
in 1987, JiM

Gray and Gianfranco Putzolu published
their now-famous five-minute rule¹⁵ for trading off
memory and I/o capacity. Their calculation compares
the cost of holding a record (or page) permanently
in memory with the cost of performing disk I/o
each time the record (or page) is accessed, using
appropriate fractional prices of RAM chips and
disk drives. The name of their rule refers to the
break-even interval between accesses. If a record

(or page) is accessed more often, it should be kept in
memory; otherwise, it should remain on disk and
be read when needed.
Based on then-current prices and performance
characteristics of Tandem equipment, Gray and
Putzolu found the price of RAM to hold a 1KB record