Hi Chris,

Sounds like you've been having fun :-)

Christopher Allan Webber <cweb...@dustycloud.org> writes:

> Christopher Allan Webber writes:
>
>> Relatedly!  User dvc in #guix on freenode suggests looking at
>> https://www.vultr.com/ which looks quite affordable and hey!  It has a
>> "custom ISO" option.  If we can convert our USB boot stick thingy
>> (presumably via xorriso) we could try generating a base server image
>> from there.  I'd prefer to have a workflow where I go from handing off
>> something made with "guix system vm-image" to some API, but maybe in the
>> meanwhile Vultr would be a lower barrier to entry.
>
> I decided to explore this a bit more today, figuring converting Guix's
> USB image to .iso couldn't be that hard.  Well, turns out it's another
> rabbit hole, and I didn't reach the end of this one yet either!

You can say that again!  ISOs and bootable CDs are an area that is often
complicated, apparently can be done in multiple ways, and is often
misunderstood or glossed over in most Internet discussions.

> Assume you've got the USB image handy (and uncompressed); then we want
> to mount it via a loopback device, but we need to get the right offset
> to find out what byte the root partition starts from.  Let's find out
> where that is.
> 
>   # We need to find out how many bytes the offset is
>   cwebber@oolong:/tmp$ parted guixsd-usb-install-0.11.0.x86_64-linux 
>   # This switches it to bytes output
>   (parted) unit B
>   (parted) print
>   Model:  (file)
>   Disk /tmp/guixsd-usb-install-0.11.0.x86_64-linux: 943718400B
>   Sector size (logical/physical): 512B/512B
>   Partition Table: msdos
>   Disk Flags: 
>   
>   Number  Start     End         Size        Type     File system  Flags
>    1      1048576B  934281727B  933233152B  primary  ext4         boot
>   
>   (parted) quit
>
> Ok, so we can take that "start" number (be sure to strip the trailing
> 'B'):
>
>   $ sudo mount -o ro,loop,offset=1048576 
> guixsd-usb-install-0.11.0.x86_64-linux /mnt/tmp

FYI, you can also mount the file on a loopback device, like I did in my
EC2 example.  If you use the -P option to losetup, it'll automatically
create a device file for the partition, which you can then mount.  That
way, there is no need to find/specify the offset.

> Now we can write it to the .iso image, packaging up all those files that
> appear in the usb image:
>
>   # flags: verbose, Rock Ridge filesystem info, output path, input directory
>   $ sudo xorrisofs -v -R -o ./guixsd-usb-install-0.11.0.x86_64-linux.iso 
> /mnt/tmp/
>   GNU xorriso 1.4.6 : RockRidge filesystem manipulator, libburnia project.
>   
>   Drive current: -outdev 'stdio:./guixsd-usb-install-0.11.0.x86_64-linux.iso'
>   Media current: stdio file, overwriteable
>   Media status : is blank
>   Media summary: 0 sessions, 0 data blocks, 0 data,  242g free
>   xorriso : UPDATE : 27100 files added in 1 seconds
>   xorriso : UPDATE : 47400 files added in 2 seconds
>   xorriso : UPDATE : 50200 files added in 3 seconds
>   [.....]
>   Written to medium : 351969 sectors at LBA 0
>   Writing to 'stdio:./guixsd-usb-install-0.11.0.x86_64-linux.iso' completed 
> successfully.
>
> Except, oops!  This won't boot.  We need to put GRUB on it.
>
> Well here's where I'm stuck for tonight.  I don't know exactly what's
> needed; maybe either the -b flag, or --grub2-mbr, or some combination.
> The man page is a little bit overwhelming (I mean, xorrisofs is clearly
> featureful, credit there):
>
>   https://www.gnu.org/software/xorriso/man_1_xorrisofs.html
>
> But how do I generate the right GRUB stuff to put there?  Can I pull it
> off the USB image?  Generate it separately?
>
> This web page is very long but appears to have the appropriate info (and
> unfortunately requires running arbitrary javascript to even render):
>
>   
> http://lukeluo.blogspot.com/2013/06/grub-how-to-2-make-boot-able-iso-with.html
>
> ... so I guess the next steps are following roughly what's described at
> the bottom of the page?

That is actually the exact webpage I was going to link to you.  What you
need to do (I think) is make the CD bootable, and the common way to do
that, apparently, is to do it via El Torito.  I've never done it,
myself, but my understanding is that that is what needs to be done.

In my opinion, unless the virtualization service provides no other easy
way to import an image for booting, I doubt that making a bootable ISO
is the simplest solution.  But it's good to experiment!

-- 
Chris

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