This article was published originally at https://kubevirt.io/2020/Live-migration.html
This blog post will be explaining on KubeVirt’s ability to perform live migration of virtual machines.
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.
The concept of live migration is already well-known among virtualization platforms and enables administrators to keep user workloads running while the servers can be moved to maintenance for any reason that you might think of like:
- Hardware maintenance (physical, firmware upgrades, etc)
- Power management, by moving workloads to a lower number of hypervisors during off-peak hours
KubeVirt also includes support for virtual machine migration within Kubernetes when enabled.
Keep reading to learn how!
Enabling Live Migration
To enable live migration we need to enable the
feature-gate for it by adding
LiveMigration to the key:
apiVersion: v1 kind: ConfigMap metadata: name: kubevirt-config namespace: kubevirt labels: kubevirt.io: "" data: feature-gates: "LiveMigration"
kubevirt-config can be edited to append “
LiveMigration” to an existing configuration:
kubectl edit configmap kubevirt-config -n kubevirt
data: feature-gates: "DataVolumes,LiveMigration"
Configuring Live Migration
If we want to alter the defaults for Live-Migration, we can further edit the
apiVersion: v1 kind: ConfigMap metadata: name: kubevirt-config namespace: kubevirt labels: kubevirt.io: "" data: feature-gates: "LiveMigration" migrations: |- parallelMigrationsPerCluster: 5 parallelOutboundMigrationsPerNode: 2 bandwidthPerMigration: 64Mi completionTimeoutPerGiB: 800 progressTimeout: 150
Parameters are explained in the below table (check the documentation for more details):
|5||How many migrations might happen at the same time|
|2||How many outbound migrations for a particular node|
|64Mi||MiB/s to have the migration limited to, in order to not affect other systems|
|800||Time for a GiB of data to wait to be completed before aborting the migration.|
|150||Time to wait for Live Migration to progress in transferring data|
Performing the Live Migration
- Virtual Machines using PVC must have a
RWXaccess mode to be Live-Migrated
- Additionally, pod network binding of bridge interface is not allowed
VirtualMachineInstanceMigrationto the cluster, indicating the VM name to migrate, like in the following example:
apiVersion: kubevirt.io/v1alpha3 kind: VirtualMachineInstanceMigration metadata: name: migration-job spec: vmiName: vmi-fedora
This will trigger the process for the VM.
When a VM is started, a calculation has been already performed indicating if the VM is live-migratable or not. This information is stored in the
VMI.status.conditions. Currently, most of the calculation is based on the
Access Mode for the VMI volumes but can be based on multiple parameters. For example:
Status: Conditions: Status: True Type: LiveMigratable Migration Method: BlockMigration
VMI.status. Once live migration is complete, a status of
Failedwill be indicated.
Migration Method field can contain:
BlockMigration: meaning that the disk data is being copied from source to destination
LiveMigration: meaning that only the memory is copied from source to destination
VMs with block devices located on shared storage backends like the ones provided by Rook that provide PVCs with ReadWriteMany access have the option to live-migrate only memory contents instead of having to also migrate the block devices.
Cancelling a Live Migration
If we want to abort the Live Migration, ‘Kubernetes-Style’, we’ll just delete the object we created for triggering it.
In this case, the VM status for migration will report some additional information:
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
Note that there are some additional fields that indicate that
Abort Requested happened and in the above example that it has
Succeded, in this case, the original fields for migration will report as
Completed (because there’s no running migration) and
Failed set to true.
What can go wrong?
Live-migration is a complex process that requires transferring data from one ‘VM’ in one node to another ‘VM’ into another one, this requires that the activity of the VM being live-migrated to be compatible with the network configuration and throughput so that all the data can be migrated faster than the data is changed at the original VM, this is usually referred to as converging.
Some values can be adjusted (check the table for settings that can be tuned), to allow it to succeed but as a trade-off:
- Increasing the number of VMs that can migrate at once, will reduce the available bandwidth.
- Increasing the bandwidth could affect applications running on that node (origin and target).
- Storage migration (check the
Infonote in the Performing the Live Migration section on the differences) might also consume bandwidth and resources.
Sometimes, a node requires to be put on maintenance and it includes workloads on it, either containers or, in KubeVirt’s case, VM’s.
It is possible to use selectors, for example, move all the virtual machines to another node via
kubectl drain <nodename>, for example, evicting all KubeVirt VM’s from a node can be done via:
kubectl drain <node name> --delete-local-data --ignore-daemonsets=true --force --pod-selector=kubevirt.io=virt-launcher
Reenabling node after eviction
Once the node has been tainted for eviction, we can use
kubectl uncordon <nodename> to make it schedulable again.
According to documentation,
--force are required because:
- Pods using
emptyDircan be deleted because the data is ephemeral.
- VMI will have
virt-handlerso it’s safe to proceed.
- VMIs are not owned by a
DaemonSet, so kubectl can’t guarantee that those are restarted. KubeVirt has its own controllers for it managing VMI, so kubectl shouldn’t bother about it.
If we omit the
--pod-selector, we’ll force eviction of all Pods and VM’s from a node.
In order to have VMIs using
LiveMigration for eviction, we have to add a specific spec in the VMI YAML, so that when the node is tainted with
kubevirt.io/drain:NoSchedule is added to a node.
spec: evictionStrategy: LiveMigrate
From that point, when
kubectl taint nodes <foo> kubevirt.io/drain=draining:NoSchedule is executed, the migrations will start.
As a briefing on the above data:
LiveMigrateneeds to be enabled on KubeVirt as a feature gate.
LiveMigratewill add status to the VMI object indicating if it’s a candidate or not and if so, which mode to use (Block or Live)
- Based on the storage backend and other conditions, it will enable
- Based on the storage backend and other conditions, it will enable