Xen BMR

Xen BMR

''' BMR on Xen Server:'''

You may have problems when doing BMR for Xen - need to understand the difference between HVM and PV:

 ''Paravirtual (PV)''
 
 Paravirtual AMIs boot with a special boot loader called PV-GRUB, which starts the boot cycle and then chain loads the kernel specified in the menu.lst file on your image.
 Paravirtual guests can run on host hardware that does not have explicit support for virtualization, but they cannot take advantage of special hardware extensions such as enhanced networking or GPU processing.

 ''Hardware Virtual Machine (HVM)''
 
 HVM AMIs are presented with a fully virtualized set of hardware and boot by executing the master boot record of the root block device of your image.
 This virtualization type provides the ability to run an operating system directly on top of a virtual machine without any modification, as if it were run on the bare-metal hardware.
 The Amazon EC2 host system emulates some or all of the underlying hardware that is presented to the guest.
 
 Unlike PV guests, HVM guests can take advantage of hardware extensions that provide fast access to the underlying hardware on the host system. 


Problems with booting up the BMR'd VM can stem from the VM being configured for PV, but run in HVM during the BMR process (since HVM is necessary to access the BMR media).



Here are steps to resolve setting back to PV, once the BMR is complete:

# Turn OFF the VM that finished the BMR.
# Find the uuid of the machine we shut down:
 xe vm-list name-label=<vm name> params=uuid
#Switch the VM from HVM to PV:
 xe vm-param-set uuid=<vm uuid> PV-bootloader=pygrub HVM-boot-policy=“”
#Take note of the VBD disk uuid:
 xe vm-disk-list uuid=<vm uuid>
.#Switch the disk VBD to be bootable:
 xe vbd-param-set uuid=<VBD uuid> bootable=true
#Turn the VM on

If you need to boot back into the BMR environment on the VM, you have to switch back to HVM.  To do so, turn off the VM, run the following, and then turn on the VM:
 xe vm-param-set uuid=<vm uuid> PV-bootloader="" HVM-boot-policy="BIOS order"

That's all. It should now boot up to the OS.

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