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--- cn:ccr:virtualizzazione:documentazione:xen_to_kvm [2009/07/07 14:38] – created chierici@cnaf.infn.it
+++ cn:ccr:virtualizzazione:documentazione:xen_to_kvm [2010/04/19 13:09] (current) – andrea.chierici@cnaf.infn.it
@@ Line 1: / Line 1: @@
+===== Migrating from XEN to KVM =====
+This small  guide describes how to migrate existing and running machines from xen (para and hvm) to kvm, and NOT VICE VERSA.
+KVM is taking over XEN and is getting rather popular, so we think that this is the time for such a guide.
+==== Before you start =====
+We assume you already have an idea on how XEN and KVM work, and how to convert a machine from one hypervisor to the other.
+Needless to say that to make the migration, you need to shutdown your machine, the migration can't be done //live//.
+This guide has been prepared using the Tier1 environment and installation system, so it's possibile that something different could be required on other sites/installations.
+We assume that the dom0 is running on a SL 5.x machine and that the VMs are SL 4.x. This does not mean that if you start from a different setup things won't work, but additional effort may be required.\\
+This procedure works both for disk-on-a-file and disk on partition approaches.
+In our environment generally disk partitions are LABELLED, so there is no need to change anything inside // /etc/fstab//. If you are using direct disk device specifications you may need to change the devices to something suitable to KVM. Generally the problem can occur with para-virtualized xen machines that use /dev/xvd instead of /dev/{hd,sd}. Pay attention to this before beginning the further steps.
+==== XEN-para to KVM ====
+Log on the node and perform some modifications:
+  * If the machine does not have already one, install a //standard// kernel (that is to say, not a xenU)
+  * Set the default kernel to the one you just installed
+  * Remove any reference to the virtual console on the kernel line //console=xvc0//
+  * modify // /etc/inittab// this way
+    * comment in tty section of inittab the line beginning with //co:// that enables the serial console
+    * uncomment all the 6 standard consoles
+  * if you don't need ldap or if you want to speed up the first boot process, add this line to // /etc/ldap.conf//
+    * bind_policy soft, **remember to add it again after you reconfigured the network**
+After all these operations, you can reboot the node using kvm instead of xen. As an example, a command line to launch a node could be something like:
+  qemu-kvm -hda disk.img -net nic,model=e1000,macaddr=00:16:3e:00:00:00 -net tap -m 2048 -vnc :0
+When the machine starts, //kudzu// will detect several hardware changes; it is quite safe to answer //yes// to all the questions and after some seconds, the node should be ready again, this time running on KVM.
+==== XEN-hvm to KVM ====
+With XEN-hvm the process is simpler. Log on the node and perform some operations;
+  * if you are using para-virtualized drivers, erase the corresponding line in /etc/modprobe.conf
+  * if you plan to use e1000 network driver, add //alias eth0 e1000// in /etc/modprobe.conf
+  * if you don't need ldap or if you want to speed up the first boot process, add this line to ///etc/ldap.conf//
+    * bind_policy soft, **remember to add it again after you reconfigured the network**
+After all these operations, you can reboot the node using kvm instead of xen. As an example, a command line to launch a node could be something like:
+  qemu-kvm -hda disk.img -net nic,model=e1000,macaddr=00:16:3e:00:00:00 -net tap -m 2048 -vnc :0
+When the machine starts, //kudzu// will detect several hardware changes; it is quite safe to answer //yes// to all the questions and after some seconds, the node should be ready again, this time running on KVM.
+===== Appendix: Mounting a disk image file to edit configurations =====
+If your VM is already turned off, you can avoid turning it on again to perform the modifications described in this page.
+You can follow this small guide.
+At first place create a device mapper for the disk image:
+//kpartx -a node1.img//
+You can check which is the device by ‘kpartx -l node1.img’.
+  # kpartx -l node1.img''<br>
+  loop0p1 : 0 8177022 /dev/loop0 63''
+Then mount the filesystem:
+  mount /dev/mapper/loop0p1 /media/xenvm
+Chroot to the the Xen VM filesystem:
+  chroot /media/xenvm
+Perform an update on the existing package to avoid any package conflicts during kernel installation and then installed the normal Linux kernel:
+  yum update
+  yum install kernel
+Exit the chroot environment:
+  exit
+Umount the disk image and remove the mapper:
+  umount /media/xenvm
+  kpartx -d node1.img
+The disk could then be used with KVM with no problems at all, both via libVirt and direct KVM/Qemu environment.