This week, I’ll be writing about how to migrate virtual machines (VMs) from one host to another in the Red Hat Virtualization (RHV) environment. I’ll be focusing on the process of migrating a single VM for this post. If you are interested in the process of migrating multiple VMs, please check out my previous post, in which I cover migrating multiple VMs from one host to another.
This article will go through the process of migrating virtual machines from one host to another in the Red Hat Virtualization environment.
Virtual machine migration
Live migration is moving a virtual machine from one physical host to another while it is active. RHVM moves memory, storage, and network connectivity of the virtual machine from the source to the target machine. Live migration is useful to support host maintenance tasks without interrupting virtual machines.
Live migration is transparent to the end user. The virtual machine remains powered on and user applications continue to run while the virtual machine is migrated to the new physical host. Clients communicating with the virtual machine should notice no more than a network pause of a few milliseconds when the transfer is complete.
For live migration to work properly, the new host must have a processor with the same architecture and features as the original host. Red Hat Virtualization helps you do that by organizing hosts into clusters. A virtual machine can only migrate to hypervisor nodes that are members of its cluster. In this way, you can ensure that virtual machines are not migrated between machines that support a different set of processor functions.
Administrators should ensure that their Red Hat virtualization environment is properly configured to support live migration before using it. The following configuration requirements are necessary for live migration of virtual machines:
- The virtual machine must be migrated to a host that is in the same cluster as the host on which the virtual machine is running. The status of both hosts should be Up.
- Both nodes must have access to the same virtual networks, VLANs, and storage domains.
- The target host must have sufficient CPU power and RAM memory to support the requirements of the virtual machine.
- The custom property cache!=none should not be set on the virtual machine. The Cache setting allows you to configure the different cache modes for the virtual machine. For live migration, virtual machine cache should be disabled to ensure consistent migration of virtual machines.
Live migration is performed using a migration network. The default configuration uses the ovirtmgmt network as both a management network and a migration network. Although each live migration is limited to a maximum throughput and there is a maximum number of migrations that can be performed simultaneously, simultaneous live migrations can saturate the shared network with administrative and migration traffic. For optimal performance, storage, migration and management networks should be separated to avoid network congestion.
Manual migration of virtual machines
Red Hat Virtualisation supports manual migration of a virtual machine between hosts connected to the same cluster. The migration can be started via the RHVM management portal. The virtual machine can be moved to a specific host or to a host automatically selected by RHV.
To manually migrate a virtual machine, use the administration portal as superuser admin :
1. In the System area, click the Virtual Machines tab. Select the virtual machine to migrate to. In the Virtual Machine Host field, specify the host on which the virtual machine will run.
2. When you have selected the virtual machine to migrate, click Migrate in the top pane. A window titled Migrate Virtual Machine(s) appears.
3. To determine to which host the virtual machine will be migrated, select the Select Destination Host check box. Select the destination node from the menu. Press OK to migrate the virtual machine.
4. On the Virtual Machines tab, select the Status field for the virtual machine. When the migration is complete, this status changes from Migrate to High.
5. Make sure the Virtual Machine Host field contains the expected destination host.
Automatic migration and migration policy
Routine management of a Red Hat Virtualization environment may require taking hosts offline for upgrades and maintenance, or distributing the load more evenly across the cluster. A cluster’s migration policy may, under certain circumstances, automatically move virtual machines running on one host to other hosts.
For example, a migration policy might specify that virtual machines running on one host should be automatically migrated to other hosts in the cluster when the original host goes into maintenance mode. Migration policies can also be used to set scheduling policies to support automatic migration of virtual machines when a certain threshold of resource usage on the host is reached.
Configuring migration policies
Virtual machine migration is a network-intensive process. RHVM copies the memory state of the virtual machine to the new host over the network. In a situation where ten or more virtual machines are running on a host, migrating all of them can take a lot of time and resources. Managers must therefore ensure that they choose the measures that are most appropriate for their environment.
NOTE: To make live migration work, RHVM copies the status of the virtual machine to the new host in real time. After completing the migration, it may be necessary to recover the state that was changed during the migration. The idea is that the final migration converges so that RHVM can pause the virtual machine for a split second and transfer the final changes to the new host. At this point, the virtual machine on the new host is turned off.
In some cases, a heavily loaded system may take a long time to converge. The migration policy also determines how Red Hat Virtualisation deals with this situation.
Red Hat Virtualization Manager automatically initiates a live migration of all virtual machines running on the host when the host goes into maintenance mode. The target host for each virtual machine is set when the virtual machine is migrated to distribute the load across the cluster.
RHVM also automatically initiates live migrations of virtual machines to maintain load balancing or power saving levels, depending on policy. With RHVM, administrators can disable automatic migration of virtual machines. You can disable manual migration of virtual machines by configuring the virtual machine to run only on a specific host. The migration policy configuration includes a failover policy configuration that defines a migration policy for virtual machines in the event of a host failure.
To configure a migration policy for the cluster through the management portal :
1. Click the Cluster tab in the system area. Select the cluster and press Edit. The Edit Cluster window appears with the title Edit Cluster. Go to the Migration Policy section to define the current migration configuration for the cluster.
2. Under Migration Policy, select the migration policy to be applied from the Migration Policy menu.
The default migration policy is the minimum downtime policy. The minimum downtime migration policy is optimized for minimum virtual machine downtime during migration, but can interrupt the migration if convergence takes too long. Other policies available: Post-copy migration, suspend workloads as needed and legacy :
– The post-copy migration policy is also optimized to achieve the smallest possible break. If the migration does not converge in time during this procedure, the system switches to post-copy. Postcopy runs the virtual machine on the target host as quickly as possible. To do this, only a portion of the virtual machine’s memory is moved to the target hosts. If a virtual machine tries to access a memory page that is not in the destination node, it generates a page error and the source node forwards the page.
– The disabling policy for on-demand workload migration supports migration under most load conditions, but under high load conditions there may be a longer pause in virtual machine operation.
– Finally, the legacy migration policy supports the migration policy used in Red Hat Enterprise Virtualisation 3.6.
3. The bandwidth listed in the Bandwidth section limits the maximum bandwidth in Mbps per host for outbound and inbound migrations. Three options are available: Auto, Standard Hypervisor and Custom.
– Automatic mode uses the throughput limit defined in the data center node network QoS. If the speed limit parameter is not defined, the minimum speed for the NICs of the source and destination nodes is determined.
– The default hypervisor mode uses the VDSM setting on the source host.
– The user mode uses the user-defined bandwidth in Mbit/s.
4. The fault tolerance policy defines a policy for migrating virtual machines in the event of a host failure. RHVM migrates virtual machines running on a host that has unexpectedly shut down or gone into maintenance mode to other hosts in the cluster.
RHV supports migrating all virtual machines with the Migrate virtual machines policy, migrating only highly available virtual machines with the Migrate only highly available virtual machines policy, or disabling the migration of virtual machines with the Do not migrate virtual machines option.
NOTE: Some additional settings are available under Advanced Features. These settings can only be configured if the Legacy migration policy is selected.
Adoption of planning policy
RHV supports the configuration of scheduling policies for the distribution of virtual machines to hosts. These policies use a combination of filters and weights to determine which host the RHVM places the virtual machine on.
RHV supports five standard planning policies: Equal_Distributed, InClusterUpgrade, None, Power_Saving, and VM_ Equal_Distributed. Each policy is associated with a set of properties that can be used to modify its behavior.
To set a scheduling policy for a cluster, do the following:
1. Click the Cluster tab in the system area. Select the cluster and press Edit.
2. The Edit Cluster window appears with the title Edit Cluster. In the planning policy section you will find the current planning policy. RHVM sets the scheduling policy to None by default. In the default configuration, a virtual machine cannot be deployed on an overloaded host. A host is overloaded if its CPU load exceeds 80% for more than 2 minutes.
3. Under Policy choice, select the policy for the cluster.
4. Each scheduling policy has its own set of configurable properties. The vm_evenly_distributed policy has z. B. The following characteristics:
- The HighVmCount property sets the maximum number of virtual machines per host. An overloaded host runs more than this maximum number. The default value is 10.
- The MigrationThreshold property adjusts the buffer before virtual machines are migrated out of the host. The default value is 5.
- The SpmVmGrace property indicates the number of virtual machines that are less on the SPM host. The default value is 5.
Then click OK to apply the policy.
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