Creating Ubuntu 12.04 Paravirtualized VM for Xen

Creating an Ubuntu VM:-

First of all, you need to create an Ubuntu VM. For that, there is no default template in XENServer 6.0. Therefore, select the "Other Install Media" template. Make sure that the VM-Name which you write should not contain any spaces. This is because we are going to use a script at later stages and it gives error if vmname contains space.

XenServer 5.5's pygrub(the bootloader for PV) is unable to read Ext4 filesystem. This could be a limitation in XenServer 6.0 as well although I am not sure about it. Therefore to be on the safer side, make sure that your /boot partition uses Ext3 or Ext2 filesystem. Here is one sample partition table which you can make. Make 3 partitions and keep the rest of space unallocated which we can allocate later.

Partition Table:-

Type: /boot              Filesystem:   Ext3/Ext2                Size:    5 GB

Type: /                     Filesystem:   Ext4/Ext3/Ext2        Size:  10 GB

Type: Swap                                                                Size: 2 * Size of RAM

Unallocated Space: XX GB

If you don't want to take advantage of Ext4, then you can format using entire disk and using filesystem type as anything except Ext4.

Once done with this, just install Ubuntu12.04 on the VM. Also install openssh-server on that machine.

rahul@ubuntu$ sudo apt-get install openssh-server

Xen uses hvc0 console. So one needs to change tty1 to hvc0. For that perform the following steps:-

step1:-

rahul@ubuntu$ sudo cp /etc/init/tty1.conf /etc/init/hvc0.conf

Step2:-

Open hvc0.conf and replace all tty1 occurences by hvc0.

rahul@ubuntu$ sudo vi /etc/init/hvc0.conf

That file looked like this on my Ubuntu12.04 VM.

Now there are a fixed set of commands which need to be run to make the VM paravirtualized. Here on this link, they have listed those commands.

Commands to run on Xen's Dom0 to make a VM PV

These commands are used(are present) in the script which we are going to run at later stage. Now it requires some parameters as input which are present in the grub.cfg file. So we will copy that part from grub.cfg and keep it in some text file with us so that we can refer to it at later stages when required.Copy these parameters from VM and keep with you so that its available when the VM is off.

We need to copy the "Default Menu Entry" and copy its entire stuff(kernel and ramdisk name) in a different text file for later use. Here is example of the grub.cfg of my machine "Ubuntu12.04". Please note that the location of kernel and initrd will change in your cases as it depends on which partition your /boot is. So just copy the default menu entry and its whole content till the start of next entry.

Xen modules are not loaded into initramfs. We can correct that by adding the entry in modules file. Enter the following command for that:

rahul@ubuntu$ sudo echo "xen-blkfront" >> /etc/initramfs-tools/modules 

rahul@ubuntu$ sudo update-initramfs -u

Shutdown the VM. Create a snapshot if you want to revert back if some error occurs.

 Now you need to SSH to XenServer ie. to Dom0 machine and configure the properties of DomU machine which you want to paravirtualize.
Perform "xe vm-list" to list all the vm's present on xen host. Identify the name of vm which you want to paravirtualize.

root@xen# xe vm-list

  Now download the script available here.

The above mentioned script contains nothing but the steps which we have pointed out above on the blog with the link to that site.

Change the execution permissions of the script.

root@openstack-xen1# chmod +x makepv.sh

Execute the script. It will ask for vmname. Provide the name of VM which you want to para-virtualize.

root@openstack-xen1# ./makepv.sh <vmname>

Then it will ask for other parameters like kernel path and kernel parameters.
Provide them from the boot_config we have earlier copied and saved in a file. Copy the parameters from that file and provide them here.
If you don't have the script, then manually execute the commands as listed in the blog-post provided above.

Once its done, you need to install xentools onto the VM which you are paravirtualizing.
So connect xen-tools.iso to the ubuntu12.04 vm and power-on the VM.
SSH to the vm and install xen-tools on it.

Steps to install xentools:-

ssh <vm_ip>
sudo mount /dev/cdrom1 /media/cdrom
cd /media/cdrom/Linux
sudo dpkg -i xe-guest-utilities_6.0.0-743_amd64.deb

If you try to access vm-console using xencenter and the default console is hvc0, then you might see the error that you are unable to type anything  there. This is one of the bugs of xencenter. You just need to close xencenter and reopen it again to get it fixed.

Reboot the VM.
You need to change the run-level of the services. For that, enter the following commands:-

ssh <vm ip address>

sudo update-rc.d -f xe-linux-distribution remove
sudo update-rc.d xe-linux-distribution defaults

Reboot the VM.

Now your ubuntu12.04 VM. Your VM is paravirtualized now.

Run xe vm-param-list command and you will see that now it uses pygrub instead of HVM.