Re: [gentoo-user] booting - I don't anystand how the (Linux) world works anymore

[Helmut Jarausch]

Thanks Rich for the explanation.

Yes, I don't use an initramfs as it's not necessary in my simple case.

Fortunately, Jeremy's hint to use  root=PARTUUID=....  works just fine  
(if one knows about it)

Helmut


On 06/25/2016 09:46:21 PM, Rich Freeman wrote:
> On Sat, Jun 25, 2016 at 2:33 PM, Helmut Jarausch <jarausch@××××××.be>
> wrote:
> >
> > I don't understand  the   'root=' option on the boot line like
> > kernel /boot/vmlinuz-4.7.0-rc4 root=/dev/sda1
> >
>
> It is pretty simple.  If you're not using an initramfs, the kernel
> attempts to find the device you list and mount it as /, and then it
> runs whatever you pass in init=, or the canned list of init locations
> it has stored inside if you don't pass that.  If you're using an
> initramfs, the kernel mounts it as / and runs its init, passing the
> root= parameter over to it.
>
> >
> > Having booted by SystemRescueCD from the cdrom device, my root
> device is
> > labelled  /dev/sda1
> >  BUT trying to use that on the kernel boot line fails (the kernel
> cannot
> > find the root file system)
> >
> > By trial and error I've found that I have to use   root=/dev/sdb1
> >
> > but if I plug in an external drive (via USB) this doesn't work any
> more.
>
> As you've no doubt figured out, the issue is that these device names
> are assigned dynamically, and if your configuration changes (which I
> suppose might even include changing the boot device depending on your
> firmware), then these names can change.  The kernel's logic is pretty
> basic and prone to these kinds of failures.
>
> >
> > So, I came up with    root=UUID=uuid_number of the root file system.
> >
> > But to my surprise I now got  a kernel panic
> > syncing: VFS: unable to mount root fs on unknown block(0,0)
> >
> > So, please tell me what I'm missing?
>
> The linux kernel doesn't have any concept of device UUIDs.  It can
> only accept valid device names.
>
> The bit you're missing is that the kernel only interprets the root=
> parameter if you're not using an initramfs.  If you're using an
> initramfs then the kernel just lets the initramfs mount root.  The
> initramfs is basically a mini linux distro that can do just about
> anything it wants, and most have a way to handle devices identified by
> UUID.
>
> I'd suggest using dracut, which seems to be the most robust initramfs
> out there.  To use it (or any other initramfs) there are basically a
> few steps:
>
> 1.  Create an initramfs image in /boot (the syntax varies by tool).
> 2.  Configure your bootloader to load the initramfs (the syntax varies
> by bootloader)
> 3.  Pass an appropriate root= line to the kernel (the syntax varies by
> initramfs implementation, but most handle the syntax you gave, as well
> as the various udev paths that are based on UUIDs and labels and
> such).
>
> For dracut #1 is:
> dracut "" version (ie dracut "" 4.7.0-rc4)
>
> For grub1 #2 find the kernel line in your grub config file, and add a
> line:
> initrd <filename>  (ie initrd initramfs-4.7.0-rc4.img)
>
> Use the same path in that as your kernel, so if your kernel has some
> kind of preceding directory path use the same for the initramfs.
>
> While you can often get away without using an initramfs, in general
> they tend to make the boot process more robust and they don't really
> have any impact on the system once it is running.  They're loaded into
> a ramfs, and they typically delete themselves out of ram right before
> execing the real init.  They just give you a LOT more options while
> booting (especially with dracut ; you'd probably be able to mount root
> off of iscsi over a vpn using it).
>
> --
> Rich
>
>
>