Comment 195 for bug 1886148

Thanks very much for the analysis, Thomas!

> There remain riddles:

> Why does the grub-mkrescue ISO not boot on V14IIL ?
> (I still wonder which other ISOs boot on that machine.)

> Why did the old Ubuntu ISOs with their invalid GPT boot on sudodus' V130
> whereas a similarly invalid GPT did not help with the 20201007.1 ISO ?

Yes, that is puzzling.

> Maybe Canonical Ltd. should ask Lenovo Group Limited for enlightenment.

I've escalated it internally to the right team, and they will follow up, but
no guarantees we'll get a meaningful answer :)

> -------------------------------------------------------------------------

> The xorrisofs option to cause GPT during the production of an Ubuntu ISO
> would be

> -appended_part_as_gpt

Thinking it through, I am happy for us to make this change from MBR to GPT.

- optical media don't care about the partition table, only the El-Torito
  boot image.
- BIOS boot of USB doesn't care about the partition table, only the MBR
- UEFI 2.0 requires systems to understand GPT
- pre-2.0 UEFI systems are mostly not guaranteed to work anyway.

I've made this switch now and confirmed the result at
<http://cdimage.ubuntu.com/ubuntu/daily-live/20201009.1>.

I do see that the GPT has an extra partition at the end; is this required
for alignment? I haven't seen such partition entries when using MBR.

$ gdisk -l ~/devel/iso/groovy-desktop-amd64.iso
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /home/vorlon/devel/iso/groovy-desktop-amd64.iso: 5736268 sectors, 2.7 GiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 8897A8AA-43A3-4BD4-A4AA-C472E4376703
Partition table holds up to 248 entries
Main partition table begins at sector 2 and ends at sector 63
First usable sector is 64, last usable sector is 5736204
Partitions will be aligned on 4-sector boundaries
Total free space is 1 sectors (512 bytes)

Number Start (sector) End (sector) Size Code Name
   1 64 5725651 2.7 GiB 0700 ISO9660
   2 5725652 5735603 4.9 MiB EF00 Appended2
   3 5735604 5736203 300.0 KiB 0700 Gap1
$

Here is the full commandline used to construct this image.

xorriso -as mkisofs -r -checksum_algorithm_iso md5,sha1 -V Ubuntu\ 20.10\ amd64 -o /srv/cdimage.ubuntu.com/scratch/ubuntu/groovy/daily-live/debian-cd/amd64/groovy-desktop-amd64.raw -J -joliet-long -l -b boot/grub/i386-pc/eltorito.img -no-emul-boot -boot-load-size 4 -boot-info-table --grub2-boot-info --grub2-mbr cd-boot-images/usr/share/cd-boot-images-amd64/images/boot/grub/i386-pc/boot_hybrid.img -append_partition 2 0xef cd-boot-images/usr/share/cd-boot-images-amd64/images/boot/grub/efi.img -appended_part_as_gpt -eltorito-alt-boot -e --interval\:appended_partition_2\:all\:\: -no-emul-boot -partition_offset 16 cd-boot-images/usr/share/cd-boot-images-amd64/tree CD1