[arch-general] Removing initrd (For use with GRUB2, LVM, GPT)

[Jonathan Vasquez]

On Mon, Jan 9, 2012 at 3:41 AM, Jan Steffens <jan.steffens at gmail.com> wrote:
> On Mon, Jan 9, 2012 at 4:58 AM, Jonathan Vasquez <jvasquez1011 at gmail.com> wrote:
>> Hello everyone,
>>
>> So I've been experimenting on removing my use of initrd and using the
>> kernel directly.
>>
>> My current setup is the following:
>>
>> GPT, LVM, GRUB2 (So I can boot my partitions that are inside the LVM).
>>
>> /dev/sda1 BIOS Boot Partition EF02 (GPT)
>> /dev/sda2 Linux LVM (named arch)
>>
>> /dev/arch/boot  - /boot
>> /dev/arch/swap - swap
>> /dev/arch/home - /home
>> /dev/arch/root - /
>>
>> When I compiled my custom kernel (from upstream, and yes I did enable
>> device mapper support compiled into the kernel, I've also experimented
>> with it as a module), the kernel fails to see the / partition.
>>
>> So I turn on my computer, BIOS starts, then GRUB2 starts, GRUB2 sees
>> the /boot partition inside of the `arch` lvm because GRUB2 has support
>> for it (insmod lvm), then when the kernel (which is inside the /boot
>> partition inside the lvm) starts, the kernel "loses" the ability to
>> find the partitions inside of the lvm. Which leads me to believe that
>> GRUB2's ability to see lvm partitions doesn't carry over to the
>> kernel.. rightfully so, 2 seperate applications. Then the kernel
>> panics and says that I need to set the correct root= parameter. The
>> parameter is set but the parameter is /dev/arch/root .. a partition
>> inside of the lvm which the kernel cannot see after GRUB2 boots the
>> kernel.
>>
>> I'm assuming this is why we need an initrd. So that the /init script
>> inside the initrd does what it needs (like `vgchange -a y` then
>> mounting /dev/arch/root as newroot and transferring control back to
>> the kernel with that new root parameter).
>>
>> I guess what I need to do is rethink my partition layout because I
>> cannot see a way to boot my root LVM partition without initrd, without
>> doing a few things.
>>
>> Here is a few drawings I've made to think about my layout:
>>
>> (Current Layout)
>> http://farm8.staticflickr.com/7016/6664419375_2ee9a7b794_b.jpg
>>
>> (Pondering Sheet)
>> http://farm8.staticflickr.com/7168/6664367587_6ca149cfb3_b.jpg
>>
>> As you can see in the second sheet, my thoughts are as follows:
>>
>> Scenario 1. GRUB 2 starts, it finds /boot inside of the lvm, that then
>> triggers the kernel to start, which then finds the root partition that
>> is on a regular partition on the hdd, therefore no dm-mod support
>> needs to be compiled directly into the kernel or to use an initrd to
>> set up the environment before hand, after that happens, the init
>> scripts (also /etc/fstab) load up dm-mod and load up /home which is
>> inside of the lvm as well.
>>
>> Scenario 2. GRUB 2 starts, it finds /boot which is now on a normal
>> partition, that then triggers the kernel which also finds / on a
>> normal partition as well, and then the kernel triggers the systems
>> init scripts, and loads up all the other partitions back into the /
>> filesystem layout.
>>
>> The point of the above set up is to minimize real partition usage, and
>> maximize lvm usage for the benefits of flexibility (involving
>> resizing, shrinking, and adding more space) without the need for
>> initrd. In term simplifying the entire system.
>>
>> There are of course other ways to simplify the system, and I'm not
>> against using initrd in any way, but it's something that I would not
>> want to use, or learn not to be dependent on.
>>
>> --
>> Jonathan Vasquez
>
> If you're not against initrd, then just keep using it. It's the
> standard way of booting, not evil, and here to stay. The kernel alone
> just isn't smart enough to boot from much more than a simple root
> partition. And if it's modularized like our stock kernel, can't boot
> alone at all (due to missing drivers).
>
> By the way, my current setup is as follows:
> sda (GPT)
> - sda1: EFI System Partition (FAT), contains GRUB2, mounted at /boot
> - sda2: swap
> - sda3: Btrfs, mounted at /
>
> Having grub and the kernels on the same partition simplifies the grub
> configuration quite a bit.

I guess my adventure to boot / without initrd has come to an end.. -_-

I was studying the mkinitcpio code.. I have to say, shoutouts to the 4
devs.. you did a great job, my code would not even be close to that.

-- 
Jonathan Vasquez