Re: ISO installer image: GPT versus MBR partitions

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

This sounds nice and clean.  Thank you!

On Tue, Apr 23, 2019 at 08:14:48PM +0200, Thomas Schmitt wrote:
> A disadvantage is that for best overall properties, this layout should
> have the ISO partition start at 512-byte block 64. This needs a second
> superblock and directory tree. About 17 MiB with Guix 0.16.0.
> 

The ISO files are offered on alpha.gnu.org only with xz compression.
I suppose most of the 17 MiB would disappear with compression.


> So programs like /sbin/isosize can tell the image size even when the
> ISO has already been copied to USB stick.
> 

Sounds like this could be used to get a checksum or maybe GPG
verification of a USB drive that supposedly contains the iso.



> Somewhat off topic:
> 
> The script also has a killer for the MBR signature in the EFI image
> file. It gets activated together with partition table erasure by
>   export MKRESCUE_SED_IN_EFI_NO_PT=extra
> It is meant for those users who are curious whether block 0 of an EFI
> partition really must look like an MBR to be accepted by EFI firmware.
> 
> It would be interesting to learn about EFIs which fail without MBR
> signature.
> 
> The specs say that there may be "compatibility code" in a EFI System
> Partition. They talk of "MBR" and understanding partitions in the EFI
> partition. I understand it as "may be there, but does not have to".
> 
> Minimal test outside of Guix ISO production:
> 
> - Install GRUB and configure it for EFI for 32 or 64 bit as your firmware
>   needs. (Both is combinable, as Debian GRUB packages demonstrate.)
> 
> - Create a minimal bootable ISO. It will not offer more than a GRUB
>   command prompt after it was successfully booted:
> 
>     mkdir minimal
>     touch minimal/empty-file.txt
> 
>     export MKRESCUE_SED_MODE=original
>     export MKRESCUE_SED_IN_EFI_NO_PT=extra
> 
>     grub-mkrescue -o output.iso minimal \
>                   --xorriso=./grub-mkrescue-sed.sh
>

This prints

florian@florianmacbook ~ [env]$ grub-mkrescue -o output.iso minimal --xorriso=./grub-mkrescue-sed.sh
grub-mkrescue: warning: Your xorriso doesn't support `--grub2-boot-info'. Some features are disabled. Please use xorriso 1.2.9 or later..

and yields no output.iso for me.  Note that xorriso actually has
version 1.5.0.

Regards,
Florian