Debian 12 Preseed and ISO Link Enhancement

[ArteChp]

Hello, thank you to all the authors and contributors of this project.
It helped me a lot to achive my goals, but ..

Describe the Enhancement

I faced some issues while building a Debian 12 (x86-64) image box for Vagrant (Libvirt):

1. By default, KVM uses the Para-virtualization driver, which requires /dev/vda instead of /dev/sda.
   Since I couldn’t find a way to set Full Virtualization in KVM, I modified preseed.cfg with the line: d-i grub-installer/bootdev string /dev/vda.
   I suggest adding a comment in preseed.cfg like the following:

# If you use paravirtualized drivers for kvm/Linux - https://www.linux-kvm.org/page/Virtio
# d-i grub-installer/bootdev string /dev/vda

2. The full Debian iso-dvd file referenced in os_pkrvars/debian/debian-12-x86_64.pkrvars.hcl is large.
   Due to my limited internet bandwidth, I exceeded the 30-minute timeout multiple times while downloading it. Switching to the lighter iso-cd file resolved the issue.
   I didn’t notice any functional difference during the installation. What is the advantage of using iso-dvd over iso-cd?

Describe the Need

1. Additional comments or instructions in preseed.cfg could save KVM users time and prevent errors during Debian installation.

2. Offering an option for a lighter ISO file, such as iso-cd, would benefit users with slower internet connections.

Current Alternative

1. For the KVM issue, I modified the grub-installer boot device to /dev/vda. I prefer the performance of Para-virtualization in KVM over Full Virtualization with VirtualBox. Perhaps there are other ways to configure KVM for Full Virtualization.

2. With a bad Internet connection, I found just an alternative to download more light weight iso file iso-cd.

Can We Help You Implement This

I understand this is a relatively small contribution, but if it’s deemed valuable, I’d be happy to push a PR myself or support someone else in implementing these changes.

[MBradbury]

I also encountered the issue with /dev/vda using qemu with Debian 12. My solution was slightly different.

I added a new variable:

variable "qemu_disk_interface" {
  type = string
  default = "virtio-scsi"
}

and passed it through to the source block:

    # On Linux machines install fails when trying to install GRUB
    # This is because it targets /dev/sda when the disk is /dev/vda
    # So set the disk interface to be scsi to ensure we get a /dev/sda
    disk_interface            = var.qemu_disk_interface

Using virtio-scsi means that the VM will be generated with /dev/sda instead of /dev/vda. virtio-scsi is slightly slower than virtio-blk, but it is not something I am too concerned about for my use-case.