HP ProLiant DL288 ISS Technology Update, Volume 9 Number 1 - Page 2

ISS Technology Update Volume 9, Number 1 How many bytes are in a GB? Storage device manufacturers and operating system developers answer this question differently. This is the reason why a number of lawsuits have sprung up around the country, typically between storage device manufacturers and disgruntled customers. For example, the capacity of a 300,000,000,000 byte drive is reported as 300 GB by storage device manufacturers but is often reported as 279.4 GB or 286,102 MB by the OS. The variation stems from different interpretations of the "prefixes" G and M. Prefixes are the set of letters that precede units such as "bit" (b) and "byte" (B). The computing industry uses prefixes like kilo (k), mega (M), giga (G), and tera (T) to describe disk storage capacity, file size, memory size, and data transmission rates (10 Gigabit Ethernet, 6 Gb/s SAS). The prefixes k, M, G, and T are from the International System of Units (SI), and they are defined as powers of ten: 103, 106, 109, and 1012, respectively. In many instances, however, the computer industry has erroneously used these SI decimal prefixes to quantify binary multiples, which are based on powers of two: 210, 220, 230, and 240 (see the note "How it started"). The error, and resulting confusion, due to the incorrect use of SI decimal prefixes for binary multiples becomes more significant for larger multiples (see Table 1-1). Disk drive manufacturers correctly use SI prefixes and power of ten values because drive capacity is not intrinsically a power of two. Although drive capacity is a multiple of 512 bytes (n29), it is not restricted to being 2n bytes. Software often reports drive and file capacity by misusing SI prefixes with power of two values. This difference has resulted in lawsuits against some storage product manufacturers since the OS may report the capacity of a 300 GB drive as "279.4 GB". To protect against legal action, manufacturers typically include unnecessary disclaimers such as "When referring to hard drive capacity, one gigabyte, or GB, equals one billion bytes and one terabyte, or TB, equals one trillion bytes." Table 1-1. The difference (error) between decimal and binary is more significant as binary multiples increase. Factor 103 106 109 1012 1015 1018 1021 1024 Correct SI usage Value 1,000 1,000,000 1,000,000,000 1,000,000,000,000 1,000,000,000,000,000 1,000,000,000,000,000,000 1,000,000,000,000,000,000,000 1,000,000,000,000,000,000,000,000 Name kilo mega giga tera peta exa zetta yotta Prefix k M G T P E Z Y Factor 210 220 230 240 250 260 270 280 Incorrect binary usage Value 1,024 1,048,576 1,073,741,824 1,099,511,627,776 1,125,899,906,842,624 1,152,921,504,606,846,976 1,180,591,620,717,411,303,424 1,208,925,819,614,629,174,706,17 6 Error 2% 5% 7% 10% 13% 15% 18% 21% How it started The size of a byte-the smallest amount of data a computer can access at once-is hardware dependent. The modern de facto standard of 8 bits per byte is partly due to the IBM System/360 architecture introduced in the 1960s and the 8-bit microprocessors introduced in the 1970s. The IBM System/360 team was the first to misuse SI prefixes to describe RAM capacity. They used the value "32k" to describe 321024 bits because it was approximately 321000. New binary prefixes In 1998, the standards organization International Electrotechnical Commission (IEC) adopted new prefixes for binary powers based on recommendations by the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST). The binary prefixes are defined in the international standard IEC 60027-2:2005, third 2