Michael Stone (12025-01-08): > For example...let's take the 18000000000000B drive discussed earlier. That's > 18TB or 16TiB. Annoying, but ok. Now that's also 18000MB but 16763MiB. And > it's 18000000MB or 17166137MiB. So if you have a display in MB and you want > to know the value in TB you move the decimal 6 places. But if you move the > decimal 6 places to get from MiB to TiB you get...the wrong answer. Does > this actually happen? Yes. All the freaking time. (A classic mashup is 1024k > blocks expressed with power of 10 M and G.) Now this next part is important: > no normal human working with files and disk space and trying to communicate > with other normal humans calculates the values in base 8 or base 16. > Communication of numbers between ordinary people generally happens in base > 10. SI prefixes are base 10. And when you munge up some stupid base 2 units > with what people want and expect to be base 10, mistakes and confusion > happen. And the benefit of all that confusion and increased cognative load > is: absolutely nothing.
The thing is, for people who _expect_ rather than _know_ about this, 10% accuracy is plenty enough. Especially since, as has been pointed, a lot of factors will conspire to reduce the space available in practice. For the people who need exact figures, on the other hand, binary units are much more convenient, not just to measure the size of memory modules: alignment requirements, maximum sizes of files and devices, size of stripes, they are all based on powers of two. Stefan Monnier (12025-01-09): > A related problem is when writing code which displays such sizes in > a human readable way, for example a "speedometer" displaying the number > of bytes per second with a limited amount of space. A common choice is > to use 3 chars for the number plus a unit, i.e. things like "254 kB/s" > or "1.5 MB/s". Now, if you use the "1024" multiplier, you get a funny > quirk when the current speed is, say 1003 kB/s, because "1003 kB/s" uses > one char too many, yet we haven't reached "1.0 MB/s" either. Well, “1.5” is too few significant digits in the first place. So, you start: 100 M 999 M 1000 M 1024 M 1.59 G 9.99 G 10.0 G 99.9 G 100 G 999 G 1000 G The switch to the next prefix does not have to be as soon as the mantissa is greater than one. Notice that the heights of mountains are in meters, not in kilometers, even for the Himalaya. It is better to go to 9999 M and only then switch to 9.77 G: more significant digits, not less readable. (Also, the people who designed the symbols for the prefixes really dropped the ball with k instead of K, but that predates memory units by a long time.) -- Nicolas George
