Recently I have been tasked with the daunting process of converting a setup of HVM enabled VMs (running on Citrix XenServer 5.6.0) into PV (paravirtualised) containers.

The constraints of the project was that:

1. The operating system must be functionally identical after the migration.
2. minimal modification to the operating system (with exception of kernel / drive mapping)

I also was allowed to change the bootloader(ie, grub) in what ever way I see fit.

However, I have attempted this, I will firstly like to show you my steps I took.

This at the moment is CentOS5.5 specific:

Steps:

1. yum install kernel-xen

   This installed: 2.6.18-194.32.1.el5xen

2. edited: /boot/grub/menu.lst changed my specs to match:

   title CentOS (2.6.18-194.32.1.el5xen)    
   root (hd0,0)
   kernel /vmlinuz-2.6.18-194.32.1.el5xen ro root=/dev/VolGroup00/LogVol00 console=xvc0
   initrd /initrd-2.6.18-194.32.1.el5xen.img
   

   Then I changed my xenserver parameters to match:

   xe vm-param-set uuid=[vm uuid] PV-bootloader-args="--kernel /vmlinuz-2.6.18-194.32.1.el5xen --ramdisk /initrd-2.6.18-194.32.1.el5xen.img"
   xe vm-param-set uuid=[vm uuid] HVM-boot-policy=""
   xe vm-param-set uuid=[vm uuid] PV-bootloader=pygrub 
   xe vbd-param-set uuid==[Virtual Block Device/VBD uuid] bootable=true
   

Some things to note, I am running a VolGroup LVM ;)

Anyways, after all these steps (which aren't much!) I boot the VM and it boots initial kernel just fine, however I am presented with this error:

Boot Screen:

device-mapper: dm-raid45: initialized v0.2594l
Waiting for driver initialization.
Scanning and configuring dmraid supported devices
Scanning logical volumes
  Reading all physical volumes.  This may take a while...
Activating logical volumes
  Volume group "VolGroup00" not found
Creating root device.
Mounting root filesystem.
mount: could not find filesystem '/dev/root'
Setting up other filesystems.
Setting up new root fs
setuproot: moving /dev failed: No such file or directory
no fstab.sys, mounting internal defaults
setuproot: error mounting /proc: No such file or directory
setuproot: error mounting /sys: No such file or directory
Switching to new root and running init.
unmounting old /dev
unmounting old /proc
unmounting old /sys
switchroot: mount failed: No such file or directory

Now my hints are that it cannot detect / because of the fact that when you change from HVM mode to PV it does something (not that obvious)

When you make a SR (storage) on a HVM, you get it mounted to the guest os as /dev/hda.

However in PV mode, this presents itself as /dev/xvda...

Could this be the answer? and if so, how the heck to I implement it??

Update:
So I have gotten a bit further in my quest, as it now detects the LVM's...

To do this, I required to recompile the xen-kernel initrd image.

Command: mkinitrd -v --builtin=xen_vbd --preload=xenblk initrd-2.6.18-194.32.1.el5xen.img 2.6.18-194.32.1.el5xen

Now when I boot I get this:

Boot Screen:

Loading dm-raid45.ko module
device-mapper: dm-raid45: initialized v0.2594l
Scanning and configuring dmraid supported devices
Scanning logical volumes
  Reading all physical volumes.  This may take a while...
  Found volume group "VolGroup00" using metadata type lvm2
Activating logical volumes
  3 logical volume(s) in volume group "VolGroup00" now active
Creating root device.
Mounting root filesystem.
mount: error mounting /dev/root on /sysroot as ext3: Device or resource busy
Setting up other filesystems.
Setting up new root fs
setuproot: moving /dev failed: No such file or directory
no fstab.sys, mounting internal defaults
setuproot: error mounting /proc: No such file or directory
setuproot: error mounting /sys: No such file or directory
Switching to new root and running init.
unmounting old /dev
unmounting old /proc
unmounting old /sys
switchroot: mount failed: No such file or directory
Kernel panic - not syncing: Attempted to kill init!