What is Live Migration

Live-migrating an instance means moving its virtual machine to a different OpenStack Compute server while the instance continues running. When you are operating a public cloud OR private cloud you need to do this for many reasons. During the live migration, users wont be aware or notified about his virtual machines are migrating from one host to another. During this process, instance wont be shutdown.

Why we need a Live Migration

Host Failure

The compute host which hosts guest virtual machines is failed. There is many reasons for this failure, such as cooling issues, Storage problems, Networking problems, Your datacenter stuck by a flood (Worst Scenario).

Maintanance mode

You need to upgrade your firmware. You need to upgrade the hardware, like increase the memory stick. OR you need to upgrade your Operating system Kernel

Optimal resource usage

Spread the guest virtual machines across the other compute host. Mitigate the noisy neighbour. Host more virtual machines on the compute host to save power. Help a VIP customer to deploy his virtual machine in a HA mode.

How a Live migration happening ?

Pre-migration

The VM1 is residing on compute-01 need to migrate to another host which is compute-02

openstack server migrate f3ef36e9-axvf-5fa45-878c-12345615bab2 --live compute-02

User commanding nova-scheduler to chose the host compute-02 as the destination host. In this case, user Identify the host himself, and he confirm that the destination host have enough resource. If the User doesnt choose the host, nova-scheduler will choose the best,optimum hypervisor for you.

Reservation

Confirm the availability of resource in compute-02. Reserve the resouce for incoming guest VM1

Iterative pre-copy

Memory is copied to compute-02 from compute-01. After that dirtied pages are iteratively copied.

Stop and Copy

Suspend the VM1 and copying remaining pages. Finally the CPU state. The suspend state doesnt mean, virtual machine is DOWN.

Commitment

The compute-02 will be the primary residence for VM1

Pitfalls

  • Guest virtual machines with intensive memory consumption are hard to workload
  • Live migration will generate a heavy load on the Network
  • Memory overconsumption
  • Different CPU architecture in destination host will interrupt the Live migration
  • NOVA wont allow any operation on the Guest VM during Live migration
  • But you can use virsh command in compute host to interact.

Abort ?

You can abort the Live migration if needed.

nova migration-list | grep <vm-id>

nova live-migration-abort <server-id> <live-migraion-id>

Futher reading