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Live Migration

Live migration is a process during which a running Virtual Machine Instance moves to another compute node while the guest workload continues to run and remain accessible.

Enabling the live-migration support

Live migration is enabled by default in recent versions of KubeVirt. Versions prior to v0.56, it must be enabled in the feature gates. The feature gates field in the KubeVirt CR must be expanded by adding the LiveMigration to it.

Limitations

  • Virtual machines using a PersistentVolumeClaim (PVC) must have a shared ReadWriteMany (RWX) access mode to be live migrated.

  • Live migration is not allowed with a pod network binding of bridge interface type ()

  • Live migration requires ports 49152, 49153 to be available in the virt-launcher pod. If these ports are explicitly specified in masquarade interface, live migration will not function.

Initiate live migration

Live migration is initiated by posting a VirtualMachineInstanceMigration (VMIM) object to the cluster. The example below starts a migration process for a virtual machine instance vmi-fedora

apiVersion: kubevirt.io/v1
kind: VirtualMachineInstanceMigration
metadata:
  name: migration-job
spec:
  vmiName: vmi-fedora

Using virtctl to initiate live migration

Live migration can also be initiated using virtctl 

    virtctl migrate vmi-fedora

Migration Status Reporting

Condition and migration method

When starting a virtual machine instance, it has also been calculated whether the machine is live migratable. The result is being stored in the VMI VMI.status.conditions. The calculation can be based on multiple parameters of the VMI, however, at the moment, the calculation is largely based on the Access Mode of the VMI volumes. Live migration is only permitted when the volume access mode is set to ReadWriteMany. Requests to migrate a non-LiveMigratable VMI will be rejected.

The reported Migration Method is also being calculated during VMI start. BlockMigration indicates that some of the VMI disks require copying from the source to the destination. LiveMigration means that only the instance memory will be copied.

Status:
  Conditions:
    Status: True
    Type: LiveMigratable
  Migration Method: BlockMigration

Migration Status

The migration progress status is being reported in the VMI VMI.status. Most importantly, it indicates whether the migration has been Completed or if it Failed.

Below is an example of a successful migration.

Migration State:
    Completed:        true
    End Timestamp:    2019-03-29T03:37:52Z
    Migration Config:
      Completion Timeout Per GiB:  800
      Progress Timeout:             150
    Migration UID:                  c64d4898-51d3-11e9-b370-525500d15501
    Source Node:                    node02
    Start Timestamp:                2019-03-29T04:02:47Z
    Target Direct Migration Node Ports:
      35001:                      0
      41068:                      49152
      38284:                      49153
    Target Node:                  node01
    Target Node Address:          10.128.0.46
    Target Node Domain Detected:  true
    Target Pod:                   virt-launcher-testvmimcbjgw6zrzcmp8wpddvztvzm7x2k6cjbdgktwv8tkq

Canceling a live migration

Live migration can also be canceled by simply deleting the migration object. A successfully aborted migration will indicate that the abort has been requested Abort Requested, and that it succeeded: Abort Status: Succeeded. The migration in this case will be Completed and Failed.

Migration State:
    Abort Requested:  true
    Abort Status:     Succeeded
    Completed:        true
    End Timestamp:    2019-03-29T04:02:49Z
    Failed:           true
    Migration Config:
      Completion Timeout Per GiB:  800
      Progress Timeout:             150
    Migration UID:                  57a693d6-51d7-11e9-b370-525500d15501
    Source Node:                    node02
    Start Timestamp:                2019-03-29T04:02:47Z
    Target Direct Migration Node Ports:
      39445:                      0
      43345:                      49152
      44222:                      49153
    Target Node:                  node01
    Target Node Address:          10.128.0.46
    Target Node Domain Detected:  true
    Target Pod:                   virt-launcher-testvmimcbjgw6zrzcmp8wpddvztvzm7x2k6cjbdgktwv8tkq

Using virtctl to cancel a live migration

Live migration can also be canceled using virtctl, by specifying the name of a VMI which is currently being migrated 

    virtctl migrate-cancel vmi-fedora

Changing Cluster Wide Migration Limits

KubeVirt puts some limits in place, so that migrations don't overwhelm the cluster. By default, it is configured to only run 5 migrations in parallel with an additional limit of a maximum of 2 outbound migrations per node. Finally, every migration is limited to a bandwidth of 64MiB/s.

These values can be changed in the kubevirt CR:

apiVersion: kubevirt.io/v1
kind: Kubevirt
metadata:
  name: kubevirt
  namespace: kubevirt
spec:
  configuration:
    migrations:
      parallelMigrationsPerCluster: 5
      parallelOutboundMigrationsPerNode: 2
      bandwidthPerMigration: 64Mi
      completionTimeoutPerGiB: 800
      progressTimeout: 150
      disableTLS: false
      nodeDrainTaintKey: "kubevirt.io/drain"
      allowAutoConverge: false
      allowPostCopy: false
      unsafeMigrationOverride: false

Bear in mind that most of these configuration can be overridden and fine-tuned to a specified group of VMs. For more information, please see Migration Policies.

Understanding different migration strategies

Live migration is a complex process. During a migration, the source VM needs to transfer its whole state (mainly RAM) to the target VM. If there are enough resources available, such as network bandwidth and CPU power, migrations should converge nicely. If this is not the scenario, however, the migration might get stuck without an ability to progress.

The main factor that affects migrations from the guest perspective is its dirty rate, which is the rate by which the VM dirties memory. Guests with high dirty rate lead to a race during migration. On the one hand, memory would be transferred continuously to the target, and on the other, the same memory would get dirty by the guest. On such scenarios, one could consider to use more advanced migration strategies.

Let's explain the 3 supported migration strategies as of today.

Pre-copy

Pre-copy is the default strategy. It should be used for most cases.

The way it works is as following:

  1. The target VM is created, but the guest keeps running on the source VM.
  2. The source starts sending chunks of VM state (mostly memory) to the target. This continues until all of the state has been transferred to the target.
  3. The guest starts executing on the target VM.
  4. The source VM is being removed.

Pre-copy is the safest and fastest strategy for most cases. Furthermore, it can be easily cancelled, can utilize multithreading, and more. If there is no real reason to use another strategy, this is definitely the strategy to go with.

However, on some cases migrations might not converge easily, that is, by the time the chunk of source VM state would be received by the target VM, it would already be mutated by the source VM (which is the VM the guest executes on). There are many reasons for migrations to fail converging, such as a high dirty-rate or low resources like network bandwidth and CPU. On such scenarios, see the following alternative strategies below.

Post-copy

The way post-copy migrations work is as following:

  1. The target VM is created.
  2. The guest is being run on the target VM.
  3. The source starts sending chunks of VM state (mostly memory) to the target.
  4. When the guest, running on the target VM, would access memory:
    1. If the memory exists on the target VM, the guest can access it.
    2. Otherwise, the target VM asks for a chunk of memory from the source VM.
  5. Once all of the memory state is updated at the target VM, the source VM is being removed.

The main idea here is that the guest starts to run immediately on the target VM. This approach has advantages and disadvantages:

advantages:

  • The same memory chunk is never being transferred twice. This is possible due to the fact that with post-copy it doesn't matter that a page had been dirtied since the guest is already running on the target VM.
  • This means that a high dirty-rate has much less effect.
  • Consumes less network bandwidth.

disadvantages:

  • When using post-copy, the VM state has no one source of truth. When the guest (running on the target VM) writes to memory, this memory is one part of the guest's state, but some other parts of it may still be updated only at the source VM. This situation is generally dangerous, since, for example, if either the target or guest VMs crash the state cannot be recovered.
  • Slow warmup: when the guest starts executing, no memory is present at the target VM. Therefore, the guest would have to wait for a lot of memory in a short period of time.
  • Slower than pre-copy on most cases.
  • Harder to cancel a migration.

Auto-converge

Auto-converge is a technique to help pre-copy migrations converge faster without changing the core algorithm of how the migration works.

Since a high dirty-rate is usually the most significant factor for migrations to not converge, auto-converge simply throttles the guest's CPU. If the migration would converge fast enough, the guest's CPU would not be throttled or throttled negligibly. But, if the migration would not converge fast enough, the CPU would be throttled more and more as time goes.

This technique dramatically increases the probability of the migration converging eventually.

Using a different network for migrations

Live migrations can be configured to happen on a different network than the one Kubernetes is configured to use. That potentially allows for more determinism, control and/or bandwidth, depending on use-cases.

Creating a migration network on a cluster

A separate physical network is required, meaning that every node on the cluster has to have at least 2 NICs, and the NICs that will be used for migrations need to be interconnected, i.e. all plugged to the same switch. The examples below assume that eth1 will be used for migrations.

It is also required for the Kubernetes cluster to have multus installed.

If the desired network doesn't include a DHCP server, then whereabouts will be needed as well.

Finally, a NetworkAttachmentDefinition needs to be created in the namespace where KubeVirt is installed. Here is an example: 

apiVersion: "k8s.cni.cncf.io/v1"
kind: NetworkAttachmentDefinition
metadata:
  name: migration-network
  namespace: kubevirt
spec:
  config: '{
      "cniVersion": "0.3.1",
      "name": "migration-bridge",
      "type": "macvlan",
      "master": "eth1",
      "mode": "bridge",
      "ipam": {
        "type": "whereabouts",
        "range": "10.1.1.0/24"
      }
    }'

Configuring KubeVirt to migrate VMIs over that network

This is just a matter of adding the name of the NetworkAttachmentDefinition to the KubeVirt CR, like so: 

apiVersion: kubevirt.io/v1
kind: Kubevirt
metadata:
  name: kubevirt
  namespace: kubevirt
spec:
  configuration:
    developerConfiguration:
      featureGates:
      - LiveMigration
    migrations:
      network: migration-network

That change will trigger a restart of the virt-handler pods, as they get connected to that new network.

From now on, migrations will happen over that network.

Configuring KubeVirtCI for testing migration networks

Developers and people wanting to test the feature before deploying it on a real cluster might want to configure a dedicated migration network in KubeVirtCI.

KubeVirtCI can simply be configured to include a virtual secondary network, as well as automatically install multus and whereabouts. The following environment variables just have to be declared before running make cluster-up: 

export KUBEVIRT_NUM_NODES=2;
export KUBEVIRT_NUM_SECONDARY_NICS=1;
export KUBEVIRT_DEPLOY_ISTIO=true;
export KUBEVIRT_WITH_CNAO=true

Migration timeouts

Depending on the type, the live migration process will copy virtual machine memory pages and disk blocks to the destination. During this process non-locked pages and blocks are being copied and become free for the instance to use again. To achieve a successful migration, it is assumed that the instance will write to the free pages and blocks (pollute the pages) at a lower rate than these are being copied.

Completion time

In some cases the virtual machine can write to different memory pages / disk blocks at a higher rate than these can be copied, which will prevent the migration process from completing in a reasonable amount of time. In this case, live migration will be aborted if it is running for a long period of time. The timeout is calculated base on the size of the VMI, it's memory and the ephemeral disks that are needed to be copied. The configurable parameter completionTimeoutPerGiB, which defaults to 800s is the time for GiB of data to wait for the migration to be completed before aborting it. A VMI with 8Gib of memory will time out after 6400 seconds.

Progress timeout

Live migration will also be aborted when it will be noticed that copying memory doesn't make any progress. The time to wait for live migration to make progress in transferring data is configurable by progressTimeout parameter, which defaults to 150s

Disabling secure migrations

FEATURE STATE: KubeVirt v0.43

Sometimes it may be desirable to disable TLS encryption of migrations to improve performance. Use disableTLS to do that:

apiVersion: kubevirt.io/v1
kind: Kubevirt
metadata:
  name: kubevirt
  namespace: kubevirt
spec:
  configuration:
    developerConfiguration:
      featureGates:
        - "LiveMigration"
    migrationConfiguration:
      disableTLS: true

Note: While this increases performance it may allow MITM attacks. Be careful.