[gentoo-user] Using UUID for root disk in grub requires initramfs?

[gentoo-user] Using UUID for root disk in grub requires initramfs?

[Adam Carter]

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This
https://wiki.gentoo.org/wiki/GRUB2/Configuration_variables

has

GRUB_DISABLE_LINUX_UUID false If true, ${GRUB_DEVICE} is passed in the root
parameter on the kernel command line.

If false, ${GRUB_DEVICE_UUID} is passed in the root parameter on the kernel
command line when an initramfs is available.

So it looks like i can't set root= to a UUID unless i use an initramfs -
can anyone confirm?

In /usr/src/linux/admin-guide/kernel-parameters.txt it has;
root=           [KNL] Root filesystem
                        See name_to_dev_t comment in init/do_mounts.c.

And in do_mounts.c it mentions PARTUUID= and PARTLABEL= but i dont know C
so don't know what to make of it.

Background is that after adding a new disk the system doesn't boot, so i'm
assuming that the /dev/sdX device names are now pointing to different
hardware, so i want to fix that by using persistent names.

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Re: [gentoo-user] Using UUID for root disk in grub requires initramfs?

[Mick]

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On Friday, 19 July 2019 10:29:09 BST Adam Carter wrote:
> This
> https://wiki.gentoo.org/wiki/GRUB2/Configuration_variables
> 
> has
> 
> GRUB_DISABLE_LINUX_UUID false If true, ${GRUB_DEVICE} is passed in the root
> parameter on the kernel command line.
> 
> If false, ${GRUB_DEVICE_UUID} is passed in the root parameter on the kernel
> command line when an initramfs is available.
> 
> So it looks like i can't set root= to a UUID unless i use an initramfs -
> can anyone confirm?

This would be correct if GRUB (with/out initramfs) happened to be the only way 
to configure Linux.  Thankfully we have more choices, in Gentoo at least.  ;-)


> In /usr/src/linux/admin-guide/kernel-parameters.txt it has;
> root=           [KNL] Root filesystem
>                         See name_to_dev_t comment in init/do_mounts.c.
> 
> And in do_mounts.c it mentions PARTUUID= and PARTLABEL= but i dont know C
> so don't know what to make of it.
> 
> Background is that after adding a new disk the system doesn't boot, so i'm
> assuming that the /dev/sdX device names are now pointing to different
> hardware, so i want to fix that by using persistent names.

You could use UUID, or partition label (if GPT is used on the disk), but by-
pass GRUB's facility to configure the UUID and use the kernel .config itself.  
For this you will have to configure and compile your own kernel.  Use this 
kernel option to specify kernel command line options:

Processor type and features -->
...

 [*] Built-in kernel command line
     (root=PARTUUID=XXXXXXXXXXXXXXXXXXXXXXXXX other_options_here)

As long as you use 'make oldconfig' for subsequent kernels the UUID will be 
retained.

-- 
Regards,

Mick

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Re: [gentoo-user] Using UUID for root disk in grub requires initramfs?

[Adam Carter]

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On Fri, Jul 19, 2019 at 8:27 PM Mick <michaelkintzios@gmail.com> wrote:

> On Friday, 19 July 2019 10:29:09 BST Adam Carter wrote:
> > This
> > https://wiki.gentoo.org/wiki/GRUB2/Configuration_variables
> >
> > has
> >
> > GRUB_DISABLE_LINUX_UUID false If true, ${GRUB_DEVICE} is passed in the
> root
> > parameter on the kernel command line.
> >
> > If false, ${GRUB_DEVICE_UUID} is passed in the root parameter on the
> kernel
> > command line when an initramfs is available.
> >
> > So it looks like i can't set root= to a UUID unless i use an initramfs -
> > can anyone confirm?
>
> This would be correct if GRUB (with/out initramfs) happened to be the only
> way
> to configure Linux.  Thankfully we have more choices, in Gentoo at least.
> ;-)
>
>
> > In /usr/src/linux/admin-guide/kernel-parameters.txt it has;
> > root=           [KNL] Root filesystem
> >                         See name_to_dev_t comment in init/do_mounts.c.
> >
> > And in do_mounts.c it mentions PARTUUID= and PARTLABEL= but i dont know C
> > so don't know what to make of it.
> >
> > Background is that after adding a new disk the system doesn't boot, so
> i'm
> > assuming that the /dev/sdX device names are now pointing to different
> > hardware, so i want to fix that by using persistent names.
>
> You could use UUID, or partition label (if GPT is used on the disk), but
> by-
> pass GRUB's facility to configure the UUID and use the kernel .config
> itself.
> For this you will have to configure and compile your own kernel.  Use this
> kernel option to specify kernel command line options:


I experimented found the following worked in /etc/default/grub;

GRUB_DEVICE="PARTUUID=d3554d49-02"

Which writes grub.cfg as;
linux /vmlinuz-5.2.0-gentoo root=PARTUUID=d3554d49-02 ro
init=/usr/lib/systemd/systemd iommu=pt raid=noautodetect

PARTUUID looked up with blkid. No initramfs required :)

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[gentoo-user] Re: Using UUID for root disk in grub requires initramfs?

[Ian Zimmerman]

On 2019-07-19 20:58, Adam Carter wrote:

> I experimented found the following worked in /etc/default/grub;
> 
> GRUB_DEVICE="PARTUUID=d3554d49-02"
> 
> Which writes grub.cfg as;
> linux /vmlinuz-5.2.0-gentoo root=PARTUUID=d3554d49-02 ro
> init=/usr/lib/systemd/systemd iommu=pt raid=noautodetect
> 
> PARTUUID looked up with blkid. No initramfs required :)

I don't get this.  blkid is userspace.  The _kernel_ needs to know what
the root fs device is, before it even starts pid 1.

With initramfs, this egg/chicken problem is hidden: the initramfs itself
is the initial root fs, and during the pivot to the real root fs
userspace programs [1] are already available.

[1]
Well, usually just 1 of them: busybox.

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Re: [gentoo-user] Re: Using UUID for root disk in grub requires initramfs?

[Mike Gilbert]

On Fri, Jul 19, 2019 at 12:37 PM Ian Zimmerman <itz@very.loosely.org> wrote:
>
> On 2019-07-19 20:58, Adam Carter wrote:
>
> > I experimented found the following worked in /etc/default/grub;
> >
> > GRUB_DEVICE="PARTUUID=d3554d49-02"
> >
> > Which writes grub.cfg as;
> > linux /vmlinuz-5.2.0-gentoo root=PARTUUID=d3554d49-02 ro
> > init=/usr/lib/systemd/systemd iommu=pt raid=noautodetect
> >
> > PARTUUID looked up with blkid. No initramfs required :)
>
> I don't get this.  blkid is userspace.  The _kernel_ needs to know what
> the root fs device is, before it even starts pid 1.

The kernel init code is able to find a device given its PARTUUID.

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/init/do_mounts.c?h=v4.19.59#n172

Re: [gentoo-user] Re: Using UUID for root disk in grub requires initramfs?

[Adam Carter]

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On Sat, Jul 20, 2019 at 2:37 AM Ian Zimmerman <itz@very.loosely.org> wrote:

> On 2019-07-19 20:58, Adam Carter wrote:
>
> > I experimented found the following worked in /etc/default/grub;
> >
> > GRUB_DEVICE="PARTUUID=d3554d49-02"
> >
> > Which writes grub.cfg as;
> > linux /vmlinuz-5.2.0-gentoo root=PARTUUID=d3554d49-02 ro
> > init=/usr/lib/systemd/systemd iommu=pt raid=noautodetect
> >
> > PARTUUID looked up with blkid. No initramfs required :)
>
> I don't get this.  blkid is userspace.  The _kernel_ needs to know what
> the root fs device is, before it even starts pid 1.
>

Sorry I was misleading by not using chronological order. The steps are;
1. Use blkid to look up the PARTUUID of the root partition
2. Add this to /etc/default/grub as GRUB_DEVICE="PARTUUID=d3554d49-02"
3. run grub-mkconfig
4. Reboot

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