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

Ady ady-sf at hotmail.com
Fri Feb 20 07:32:55 PST 2015

> I wrapped the essential code into a standalone program for
> playing with a few numbers:
What about '-h 255 -s 63 , ISO size 1085736960' ?
> I expect the average waste to be a bit less than 16 MB.
Yes, but slightly misleading. For example, the "16MB average" is 
relevant for 255/63.

Some user reading the code might not know the whole reasoning for:
> +       (Can waste nearly 32 MB. Should be done on user request only.)
I would suggest instead:

+       (Could waste up to a maximum of nearly 32 MB in certain cases.)

Note that I also removed the second phrase, "Should be done on user 
request only", as I disagree.

IMO, users should not be left with incompatible isohybrid images. For 
example, the reason for an image to fail in some VM should not need to 
be searched in this Syslinux Mailing List (a task that is not so easy 
to do). Beta testers of ISO images should not need to report to their 
respective distros that the image is failing under some situation. 
Maintainers should not need to burn their brains about the reasons. 
Users should not blame the distro (without even reporting the problem).

We are talking about an average padding of less than 2% for realistic 
cases, and that's just for the minimal size (around 1GiB). If users are 
required to download images of 3GB, adding 16MiB (on average) is less 
than 1%.

Additionally, we are talking about an average of 16MiB of padding for 
"un-padded" images. Current (as of 6.03) isohybrid images are already 
padded (with a different algorithm), so the additional (differential) 
"waste" produced by this patch is (much) less than 32MiB, and less than 
16MiB on average.

In other words, I would suggest using a more accurate wording, and 
implementing the correction, instead of leaving this issue on the air. 
Realistically, I would not expect distro maintainers to edit 
isohybrid.c so to make use of the patch; they will use what is already 
there by default. Let's see this issue solved.

Finally, let's not forget that for an image size smaller than 1GiB, the 
default geometry of 64/32 will not produce any additional waste 
comparing to the current version.

BTW, it would be nice to see TAILS testing this patch and reporting 
some feedback.

Thank you,
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