Linux – Is it possible to mount a subdirectory as the root file system at kernel startup

bootboot-loaderchrootkernellinux

I'm trying to install a linux from sources inside my home directory(as oposed to creating a dedicated partition). The basic system(glibc, kernel and init) is installed at $HOME/lfs/root. I'm booting this system using grub installed inside my usb drive, here is the relevant part in grub.cfg(I have to change grub root to hd1 since hd0 is my usb stick):

menuentry "Custom linux HD" {
set root='(hd1,1)'
linux /home/thiago/lfs/root/boot/bzImage root=/dev/sda1/home/thiago/lfs/root ro
}

It seems only devices are valid parameters for 'root='. Is there any way to work arround that? I don't want to mess with my current partitions.

Best Answer

I did not test it, but in theory (with a recent distribution and kernel) you can choose to use an initramfs or initrd. The kernel will first run with the initramfs as root, and run its boot script. The last thing the boot script does should be to call pivot_root on the mounted "real" root, and then run the real /sbin/init.

If you can get an initramfs or initrd working, maybe you can tweak the boot script do do the pivot_root in the subdirectory that you want ?

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EC2 issue using PV-Grub and custom kernel: error 21 selected disk does not exist

This is embarrassing but after two days of bashing my head on this, I think I solved it myself.

My grub menu file had commands like root(hd0,0) instead of root (hd0,0). The space is essential, and all my attempts left it out! I found this out by discovering that leaving the root line off and specifying the full path like kernel (hd0,0)/boot/vmlinuz.... worked. The lack of space did not cause any kind of parsing error, but it seems to have just been ignored. As a result it really didn't matter what values I gave it, it was just going on defaults anyway, hence the same result with the line left out entirely.

I still don't have a booting system, but I'm past the problem with grub now and onto kernel woes. Hopefully anybody else who runs into the same problem will find this solution helpful!

Grub2 does not find /boot on RAID1 md0 device

RAID is still one of the gray areas of bootloaders IMHO.

I recently built a RAID1 system and after a few hours trying to get LILO/GRUB/GRUB2 to detect my raid i gave up and just told it to use the first partition of the first HDD detected and made sure that if a HDD failed the next HDD was already lined up with the correct MBR/bootloader ect...

So what it does is it boots, grabs the kernel and initfs off the first HDD (no raid) and then boots the kernel and leaves all the RAID stuff to the kernel. Because GRUB/LILO do not physically write to the drives this wont damage them.

Basically i just ignored RAID all together for the bootloader stage.

the kernel needs to re-assemble the raid arrays even if grub does it first. there's no real reason for grub to need to be raid aware for a RAID1 system unless a drive fails during boot.

P.S. You dont need to raid0 SWAP, this ability is already in the kernel. Just set the priority for both swap devices to 1 in FSTAB

/dev/sda2         none                    swap  sw,pri=1        0 0
/dev/sdb2         none                    swap  sw,pri=1        0 0
ect....

And if a single swap drive fails during normal operations there's a very good chance your system will fail. (you can raid1 swap, just not from fstab like above)

Best Answer

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EC2 issue using PV-Grub and custom kernel: error 21 selected disk does not exist

Grub2 does not find /boot on RAID1 md0 device

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