Re: arm64 and isohybrid

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

> OK, I've just built and done a very quick test with an arm64 netinst
> build.

Does it boot ?
Are the partitions mountable ?


>    3          313344          313343   0 bytes     0700  Gap1

This looks strange.
What do you get from

  xorriso-1.3.9 -indev tmp.iso -report_system_area plain

The name "Gap1" seems to stem from libisofs. These partitions
are supposed to fill gaps in the partition layout of grub-mkrescue.
But zero sized gaps are not really intended targets.


> I've tried both with and without the "-partition_offset all" and it
> (obviously) affects the partition alignments and the overall image
> size, but otherwise the images produced *look* ok apart from the
> odd-looking zero-sized 3rd partition. It's a cosmetic issue only, but
> I'm curious why it's there. :-)

I assume you mean -partition_cyl_align, not -partition_offset.

It is a relict of MBR, its quirks, and its urban legends.
SYSLINUX (resp. hpa) emphasizes that isohybrid images have
their partitions aligned to full cylinders, preferrably to
64 heads of 32 sectors or to 255 heads of 63 sectors.
It seems to be about assumptions made by older BIOSes.

UEFI 2.4 does not have hard alignment requirements for partitions.
But there are some "should" statements in 5.3.1 GPT Overview,
which propose to align to physical block size (e.g. 4096)
and state that alignment to 1 MiB fulfills this proposal
with "most common physical block sizes and RAID stripe sizes".

Currently the alignment granularity of xorriso -as mkisofs is
controlled by -partition_hd_cyl and -partition_sec_hd which
are subject to MBR restrictions.
It has to be tested whether
  -partition_hd_cyl 64 -partition_sec_hd 32
keeps up the alignment for images larger than 1 GiB (1024
cylinders).


Have a nice day :)

Thomas