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

[Ady]

> 
> Note that the Tails 1.4~rc1 ISO image that recently brought this
> problem back on our radar is smaller than 1 GiB, so the hypothesis
> that 1 GiB would be a limit that triggers some condition doesn't seem
> to hold.
> 
> Cheers!
> --
> intrigeri
 
 
The "limit" is indeed around 1GiB, indirectly.

The heads and sectors_per_track values recommended up to 1GiB (after 
applying the isohybrid tool) are 64 and 32, respectively. With this 
pair of values, the resulting isohybrid image should be a multiple of 
2048.

When the size is bigger than 1GiB, the recommended values for heads and 
sectors_per_track are 255 and 63, respectively (the reasoning has been 
discussed in the past, so I won't repeat it). With this pair of values, 
you might find the problem we discussed during 2015Feb.

If you are already using 255 / 63 for all images, whether they are 
smaller or bigger than 1GiB, then you might be triggering the problem 
independently of the size of the image.

If the ISO image is smaller than 1GiB but "close-enough' to 1GiB, then 
the resulting isohybrid image might be bigger than 1GiB, and the 64 / 
32 pair should not be used (I have not specifically calculated the 
potential range of size values).

Additionally, the alternative partitioning schemes being introduced 
when transforming the "normal" ISO image into an isohybrid image might 
need to be considered. First, the resulting size might be different 
(thus, read my prior comments about having an ISO image close-enough to 
1GiB). Second, under certain circumstances the partition alignment 
(both start and ending ones) of the resulting isohybrid image _might_ 
need to be 1MiB (and not just 2048 bytes). This is not relevant for 
(virtual) optical media, but under other circumstances... BIOS and UEFI 
implementations, different partitioning tools, different versions of... 
Too long and boring to comment about it.

I have read enough comments from *users* with enough confusion about 
mkisofs (cdrtools) / genisoimage / isohybrid and xorriso (not to 
mention their comments about auxiliary tools when _trying_ to transfer 
an ISO/isohybrid image onto a USB drive with different methods). In 
these first days of Debian Jessie, I don't know whether TAILS will 
still be using the isohybrid tool of Syslinux, or the isohybrid 
features of xorriso.

Perhaps Thomas might be interested / curious to evaluate the image for 
TAILS 1.4-RC (before and/or after applying the isohybrid tool) 
regarding this issue.

Regards,
Ady.
 
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