[Xen-users] Newbie question: Why does Xen boot fail with initrd but succeed with initramfs?

[Yasir Assam <list1@xxxxxxxxxxxxxxx>]

I'm new to Xen. It's taken me a few days (!) just to get xen to boot. I 
kept getting the following error:
Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)
even when I used an initrd image. In the end I tried using mkinitramfs, 
and the resulting image worked.
Why should this be? Nearly every howto I read suggests mkinitrd, yet 
only mkinitramfs works on my system.
The following are the details of what I did:

-------------------------------------------------------------------------------------
I downloaded the xen-unstable source and built it using:

# make world
# make install

and I added the following to the grub menu:

title           Xen 3.0 / XenLinux 2.6
root            (hd0,1)
kernel         /boot/xen-3.0-unstable.gz dom0_mem=262144 noreboot
module         /boot/vmlinuz-2.6.16-3-xen root=/dev/sda2 ro console=tty0

For reference, this is the grub entry for my default kernel (not Xen) - 
I use Debian etch with a custom-built kernel:
title           Debian GNU/Linux, kernel 2.6.17.4-eir
root            (hd0,1)
kernel          /boot/vmlinuz-2.6.17.4-eir root=/dev/sda2 ro
initrd          /boot/initrd.img-2.6.17.4-eir
savedefault
boot

When I reboot and choose Xen from the grub menu, it crashes. The 
following are the last few lines in the log (I had to type these by hand 
- I don't konw how else to get the log - I don't think I have the 
hardware needed to set up a terminal via serial link):
md: Autodetecting RAID arrays
md: autorun ...
md: ... autorun DONE.
VFS: Cannot open root device "sda2" or unknown-block(0,0)
Please append correct "root=" boot option
Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)
I read this list and thought that maybe it's because the kernel doesn't 
have the correct SATA driver. I think I need module sata_nv, and this is 
listed as a module in the config file /boot/config-2.6.16.13-xen.
I also read somewhere that I have to use mkinitrd to create an initrd 
file otherwise modules won't be loaded, so I exectued the following 
commands:
# depmod 2.6.16.13-xen
# mkinitrd -o /boot/initrd.img-2.6.16.13.xen 2.6.16.13-xen

and I also added the following line to the Xen grub entry:

module          /boot/initrd.img-2.6.16.13.xen

but still I got a crash with the following lines:

md: Autodetecting RAID arrays
md: autorun ...
md: ... autorun DONE.
RAMDISK: cramfs filesystem found at block 0
RAMDISK: Loading 5184KiB [1 disk] into ram disk... done.
Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)
Finally, I ran the following:

# depmod 2.6.16.13-xen
# mkinitramfs -k -o /boot/initramfs.img-2.6.16.13.xen 2.6.16.13-xen

and then added the follwing to grub:

module          /boot/initramfs.img-2.6.16.13.xen

and Xen successfully booted on restart.
------------------------------------------------------------------------------

Thanks,
Yasir

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