boot an alternate kernel

[Robert Heller]

I have always installed Linux using a separate /boot file system.  Initially 
because grub1 had no clue about LVM.  I was a long time used of the free 
versions of RHEL (Whitebox, CentOS), which don't allow for major version 
upgrades in place, so when I made those upgrades, I would create a fresh root 
file system volumn and do the fresh install and use the same /boot (and /home, 
etc.) file systems.  I would end up with a "dual" boot system and use that to 
migrate all of my various system configurations, eventually mounting the old 
root readonly as a reference in case some was not right, etc.  Trying to 
manage two linux installs, each with its own /boot can be problematical when 
the two installs don't know about each other.  If they share a /boot file 
system, there is only one grub install and one grub configuration.  Ubuntu is 
a little wacky in that it might try to set things up to use one install's 
kernel with the other's root file system (and the Ubuntu grub update does not 
really like CentOS's kernel file naming system -- I had to hack things to  
make it behave properly).


At Thu, 26 May 2022 20:39:39 +0200 "Ubuntu user technical support,? not for general discussions" <ubuntu-users at lists.ubuntu.com> wrote:

> 
> On Thu, 2022-05-26 at 20:31 +0200, Ralf Mardorf via ubuntu-users wrote:
> > On Thu, 26 May 2022 20:23:41 +0200, Ralf Mardorf wrote:
> > > > Should it work from only one partition or should it work from either
> > > > partition?  
> > 
> > > You probably installed GRUB 2 times. One is the GRUB used by the boot order
> > > of your BIOS settings, the other GRUB is unused. You are likely booted
> > > to the wrong install and generating the menu.cfg for the unused GRUB.
> > 
> > In your case it's likely possible that you can use both GRUB
> > bootloaders, but you need select one bootloader by the BIOS' setting
> > boot order and you need to update the menu.cfg of both GRUB.
> 
> IOW actually you need to update only one menu.cfg. The one of the disk
> that is selected by the 1st drive of the BIOS boot order. To do this you
> need to run the install of this GRUB, not the install of the unused
> GRUB. It's not possible to do this from the other install by a
> systemd-nspawn or by a "regular" chroot. Actually you could copy the
> menu.cfg from one to the other install.
> 
> 
> 

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