XenServer Converting HVM to Paravirtualised
- by Karl Kloppenborg
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:
The operating system must be functionally identical after the migration.
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:
yum install kernel-xen
This installed: 2.6.18-194.32.1.el5xen
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!
