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--- unix:virtualization:techniques [2020/02/19 15:15] – rodolico
+++ unix:virtualization:techniques [2022/05/08 17:52] (current) – rodolico
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 Following are just some notes on how to "do things" with virtuals, especially converting from one form to another
+===== Migration =====
+One nice thing about some hypervisors it the ability to move a virtual machine (vm) from one physical machine to another. There are two main ways to do this, shutting down the running vm, or doing it while the vm is still running.
+In the following sections, //source// is the hypervisor moving from, //target// is the hypervisor being moved to, and 'vm' is the virtual machine being moved.
+==== Shutting Down and moving ====
+This method has the least limitations, but takes longer and may require significant manual intervention. The configuration of the source and target may be different, and the block devices do not need to be on a shared system. It is even feasible to have Xen running on the source and KVM running on the target, in theory.
+  - Copy the vm configuration from //source// to //target//
+    - Modify the configuration on //target//
+      - If network bridges are different, either create the network bridge on the target, or change configuration on the //target//
+      - Perform any additional changes to the configuration file to adapt to //target// hypervisor. **Note** if source and target use different hypervisors and/or managers, this may require a complete rewrite of the configuration on //target//
+  - Shut down the vm on //source//
+  - Move all block devices to //target//.
+    - If iSCSI, NFS, or some other network shared storage, this may be as simple as mounting on //target//, and can be done before shutting down vm.
+    - If this is a physical partition,Logical Volume, or file on //source//, you must copy to //target// while the vm is down (or, snapshot and lose changes over the migration period)
+  - Bring the vm up on //target//
+==== Live Migration ====
+Live Migration rapidly migrates a running vm from source to target, in many cases with no downtime detectable by the end user.
+This generally has some limitations imposed. For example, migrating a running Xen machine must have Xen on both //source// and //target//, and the version of Xen must be very close on both machines. Same with KVM and virtlib. The CPU's on both machines should be similar also.
+But, if you have compatible systems, you can migrate machines with little or no impact on end users, either as precursor to maintenance, or simply to manually load balance your hypervisors.
+=== Limitations ===
+  * Both hypervisors must be running compatible hypervisors.
+    * Should be same hypervisor family (xen, kvm, or a manager like virtlib that knows both)
+    * Hypervisor versions should be the same, though this will generally work within major versions, ie v7.1 and v7.5 should be able to work together.
+  * Block devices should be on shared storage.
+    * Usually uses iSCSI or a file on an NFS share
+    * Paths to block devices on //source// and //target// must be exact
+    * //source// and //target// should never use block devices simultaneously.
+      * //source// should shut down vm, first, freeing up block device,
+      * //target// may now bring up block devices
+  * Network definitions used by vm configuration must be accessible with same name on both //source// and //target//.
+    * //source// and //target// should be on same subnet
+  * //source// root user must have ssh access to //target//
+    * It is preferred if //source//.root can log into //target//.root without password.
+=== Procedure ===
+  - Ensure root access from //source// to //target// (required for Xen, preferred for all others)
+    - **Xen** <code bash>ssh target 'ls'</code> should give you a listing of all files in /root on //target//
+    - **virtlib** issue the command <code bash>virsh -c qemu+ssh:%%//%%target/system list</code> should return a list of running virtuals on //target//
+  - Verify compatibility
+    - Ensure hypervisor compatibility between //source// and //target//
+    - Ensure network bridges with same name on //source// and //target//
+    - Ensure path to block devices same on //source// and //target//
+  - Issue command for migration from //source//. **Note** you can use the running virtual number instead of it's actual name in many cases
+    - **Xen** - <code bash>xl migrate vm targetmachine</code>
+    - **KVM** - unknown
+    - **virtlib** - <code bash>virsh migrate --live //vm// qemu+ssh://target/system</code>
 ===== Move VirtualBox images to KVM/Xen =====