Instance migration in OpenStack can be used to redistribute VM loads or for maintenance and comes in two types - nova migrate and nova live-migration. Nova migrate shuts down instances during migration and does not allow target host selection, while nova live-migration suspends instances to minimize downtime but requires shared storage and has additional requirements. Administrators should use nova live-migration when choosing the target host is important or to minimize downtime, and nova migrate when not choosing the host or if configuration prevents live-migration.

1. Instance Migration

2. Migration
Can be used for?
 useful when a compute host requires maintenance.
 useful to redistribute the load when many VM instances are
running on a specific physical machine.
Can NOT be used for?
 Modifying instance properties such as CPU/RAM

3. Migration Types (From Documentation)

4. Each option has requirements and restrictions
For example:
 You can't live-migrate without shared storage.
 You can't live-migrate if you have a config-drive enabled.
 You can't select the target host if you use the nova migrate
(non-live) command.

5. Migration Commands
Openstack has two commands specific to virtual machine migration:
nova migrate $UUID
nova live-migration $UUID $COMPUTE-HOST

6. nova migrate $UUID
The nova migrate command shuts down an instance to move it to
another hypervisor.
 The instance is down for a period of time and sees this as a regular
shutdown.
 It is not possible to specify the compute host you want to migrate the
instance to. (there are workarounds to do it the dirty way, however).
 This command does not require shared storage, the migrations can take a
long time.

7. nova migrate $UUID (Cont.)
The nova migrate command shuts down an instance to move it to
another hypervisor.
 The Openstack cluster chooses the target hypervisor machine based on the
free resources and availability.
 The migrate command works with any type of instance.
 The VM clock has no issues.

8. nova live-migration $UUID $COMPUTE-HOST
The nova live-migration command has almost no instance downtime.
 The instance is suspended and does not see this as a shutdown.
 The live-migration lets you specify the compute host you want to
migrate to, however with some limitations. This requires shared
storage, instances without a config-drive when block storage is used,
or volume-backed instances.

9. nova live-migration $UUID $COMPUTE-HOST (Cont.)
The nova live-migration command has almost no instance downtime.
 The migration fails if there are not enough resources on the target
hypervisor
 The VM clock might be off to prevent issues.

10. when to use which option
 If it is important to choose the compute host or to have very little
downtime you need to use the nova live-migration command.
 If you don't want to choose the compute host, or you have a
configdrive enabled, you need to use the nova migrate command.
 If you need to specify the compute host and you have a config-drive
enabled, you need to manually migrate the machine, or use a dirty
trick to fool nova migrate.