[syslinux] isohybrid and ISO images whose size is not a multiple of 2048 bytes vs. VirtualBox

Ady ady-sf at hotmail.com
Thu Feb 19 00:28:43 PST 2015

> Hi,
> Ady wrote:
> > I fear we would get slightly off-topic.
> Yep. Since the TAILS ISO is supposed to not grow over 2 GiB
> for now, it seems best to just use -h 128 -s 32 in order to
> stay near to traditions.
> -----------------------------------------------------------
> Summary:
> The alignment to cylinder size is a core feature of isohybrid.
> If -h multiplied by -s is not divisible by 4, then ISO image
> files may emerge from isohybrid which are not aligned to
> 2048 bytes. This is bad if the file shall be used as
> virtual DVD-ROM.
> The waste caused by a particular cylinder size depends on
> the size of the ISO input image.
> It is well possible to automatically determine a cylinder size
> for minimum waste while keeping it divisible by 2048.
> But it would probably produce non-traditional sizes.
> (Whatever the importance of a traditional size is.)
> It might be beneficial for the community if everybody tests
> -h 252 -s 63 whether it suits as replacement of the advise
> to use -h 255 -s 63 with large ISOs.
> -----------------------------------------------------------
> Have a nice day :)
> Thomas
I am not so sure that using a different pair of '-h' and '-s' values 
would be better than using 255/63. Most BIOS and most partition editors 
would assume this geometry. Using a different geometry might help when 
using the resulting isohybrid image as virtual optical media, but might 
also reduce the chances of successfully booting in some BIOS systems 
with non-optical media.

Additionally, what do you mean with "non-traditional sizes"? Or, in 
other words, what would constitute a traditional size for the resulting 
isohybrid image?

I wonder whether my suggestion for the algorithm is inadequate, or 
whether it would modify in some way any other condition already 
included in the isohybrid source code. If it does, then perhaps it 
would be possible to combine them?

If different "preferred" conditions go against each other, perhaps 
there should be some argument for the user to choose which conditions 
are more relevant / important for his goal / needs?

I would tend to think that the resulting isohybrid image size should be 
a multiple of 2048 bytes (independently of the command-line parameters 
chosen by the user), and that the isohybrid tools should try to match 
this condition. What currently-existing condition would not be achieved 
in such case?

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