[ale] SI Prefixes Was: A petabyte here, a petabyte there ...

[Chuck Huber]

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You know, in all this talk about large amounts of data, it reminded me
of the difference between what we (in the computer industry) call a
gigasomething (2^20 somethings) and what a gigasomething really is (10^6
somethings).

It turns out that in 1998, the IEC approved as a standard names and
symbols for prefixes of binary multiples.

    Factor    Name    Symbol  Origin                 Derivation
    2^10      kibi    Ki      kilobinary (2^10)^1    kilo (10^3)^1
    2^20      mebi    Mi      megabinary (2^10)^2    mega (10^3)^2
    2^30      gibi    Gi      gigabinary (2^10)^3    giga (10^3)^3
    2^40      tebi    Ti      terabinary (2^10)^4    tera (10^3)^4
...

So one mebibyte = 1 MiB = 2^20 B = 1048576 B
while one megabyte = 1 MB = 10^6 B = 10000000 B

There's more documentation and a brief history at
http://physics.nist.gov/cuu/Units/binary.html

In the history, Bruce Barrow points out that this has caused quite a bit
of confusion.  The manufactures of storage devices us the term megabyte
to mean 1,000,000 bytes while manufactures of memory use the term to
mean 2^20 = 1,048,576 bytes.

I'm curious as to when a "df -h" and a "ls -lh" will report ki Mi and Gi
suffixes on sizes.

Speaking of rounding significant digits...
Sometime in the 60's, the mayor of a small town in Texas decreed that a
value of 3 would be used for pi to ease calculation effort.  He
abandoned this idea when some local engineers pointed out that if he
allowed his decree to stand, people would die because of it.

Enjoy,
    - Chuck
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