how big is the cd image

[Jonathan Marsden]

Paul,

On Thu, May 10, 2012, at 08:14 PM, paul sutton wrote:

> According to ubuntu  the cd image is 721.7 MB (721,727,488 bytes) in
> size according to windows it is 688mb (721,727,488 bytes) in size
> (well a recorded cd is that size)
>
> So which one of these is correct, given that the image needs to fit on
> a normal 700mb cd-r otherwise it needs a dvd

Both are "correct" -- they just use different definitions of "MB"!

Ubuntu is using the ISO and disk manufacturer standard that 1MB =
1000000 bytes.  Windows is (I think) using the "computer industry"
definition of the "binary MB", which ISO calls a "MibiByte", in which
1MB (or 1MiB per ISO) = 2^10 = 1024 * 1024 = 1048576 bytes.

The image is under 700 "computer industry" (binary) MB, so it will fit
on a CD-R blank.

In case you care, the official Ubuntu policy on units is documented at

  https://wiki.ubuntu.com/UnitsPolicy

To verify the size of a CD-R blank, in Ubuntu or Lubuntu you can install
wodim by doing

  sudo apt-get install wodim

and then put your CD-R blank in the drive, and do

  wodim -atip |grep "lead out"

The number right after the : in the output line, outside of any
parentheses, is the number of 2048 byte allocation units available on
the CD-R.  On one I just tested here, it output

  ATIP start of lead out: 359846 (79:59/71)

so the number of clusters is 359846, which means the total bytes
available to write data to on that CD-R blank is given by

  echo $((359846 * 2048))

which outputs 736964608, which is larger than the size in bytes of the
CD image you mentioned, which was 721727488.

Bottom line: it fits, don't worry so much :)

Jonathan
-- 
  Jonathan Marsden
  jmarsden at fastmail.fm