Chapter 10. Migrating virtual machines

10.1. How migrating virtual machines works

The essential part of virtual machine (VM) migration is copying the XML configuration of a VM to a different host machine. If the migrated VM is not shut down, the migration also transfers the state of the VM’s memory and any virtualized devices to a destination host machine. For the VM to remain functional on the destination host, the VM’s disk images must remain available to it.

By default, the migrated VM is transient on the destination host, and remains defined also on the source host.

You can migrate a running VM by using live or non-live migrations. To migrate a shut-off VM, you must use an offline migration. For details, see the following table.

Table 10.1. VM migration types
Migration type	Description	Use case	Storage requirements
Live migration	The VM continues to run on the source host machine while KVM is transferring the VM’s memory pages to the destination host. When the migration is nearly complete, KVM very briefly suspends the VM, and resumes it on the destination host.	Useful for VMs that require constant uptime. However, VMs that modify memory pages faster than KVM can transfer them, such as VMs under heavy I/O load, cannot be live-migrated, and non-live migration must be used instead.	The VM’s disk images must be accessible both to the source host and the destination host during the migration. (1)
Non-live migration	Suspends the VM, copies its configuration and its memory to the destination host, and resumes the VM.	Creates downtime for the VM, but is generally more reliable than live migration. Recommended for VMs under heavy memory load.	The VM’s disk images must be accessible both to the source host and the destination host during the migration. (1)
Hybrid migration	Combines live migration and non-live migration. You suspend the source VM during live migration, which prevents additional dirty memory pages from being generated. As a result, the migration is significantly more likely to complete.	Recommended for example when live-migrating a VM that uses very many vCPUs or a large amount of memory, which prevents the migration from completing. Based on guest workload and the number of static pages during migration, a hybrid migration might also cause significantly less downtime than a non-live migration.	The VM’s disk images must be accessible both to the source host and the destination host during the migration. (1)
Offline migration	Moves the VM’s configuration to the destination host	Recommended for shut-off VMs and in situations when shutting down the VM does not disrupt your workloads.	The VM’s disk images do not have to be available on a shared network, and can be copied or moved manually to the destination host instead.

(1) To achieve this, use one of the following:

Storage located on a shared network
The --copy-storage-all parameter for the virsh migrate command, which copies disk image contents from the source to the destination over the network.
Storage area network (SAN) logical units (LUNs).
Ceph storage clusters

Additional resources

10.2. Benefits of migrating virtual machines

Migrating virtual machines (VMs) can be useful for:

Load balancing: VMs can be moved to host machines with lower usage if their host becomes overloaded, or if another host is under-utilized.
Hardware independence: When you need to upgrade, add, or remove hardware devices on the host machine, you can safely relocate VMs to other hosts. This means that VMs do not experience any downtime for hardware improvements.
Energy saving: VMs can be redistributed to other hosts, and the unloaded host systems can thus be powered off to save energy and cut costs during low usage periods.
Geographic migration: VMs can be moved to another physical location for lower latency or when required for other reasons.

10.3. Limitations for migrating virtual machines

Before migrating virtual machines (VMs) in RHEL 8, ensure you are aware of the migration’s limitations.

Live storage migration cannot be performed on RHEL 8, but you can migrate storage while the VM is powered down. Note that live storage migration is available on Red Hat Virtualization.

Migrating VMs from or to a session connection of libvirt is unreliable and therefore not recommended.
VMs that use certain features and configurations will not work correctly if migrated, or the migration will fail. Such features include:
- Device passthrough
- SR-IOV device assignment
- Mediated devices, such as vGPUs
A migration between hosts that use Non-Uniform Memory Access (NUMA) pinning works only if the hosts have similar topology. However, the performance on running workloads might be negatively affected by the migration.
The emulated CPUs, both on the source VM and the destination VM, must be identical, otherwise the migration might fail. Any differences between the VMs in the following CPU related areas can cause problems with the migration:
- CPU model
  - Migrating between an Intel 64 host and an AMD64 host is unsupported, even though they share the x86-64 instruction set.
  - For steps to ensure that a VM will work correctly after migrating to a host with a different CPU model, see Verifying host CPU compatibility for virtual machine migration.
- Firmware settings
- Microcode version
- BIOS version
- BIOS settings
- QEMU version
- Kernel version
Live migrating a VM that uses more than 1 TB of memory might in some cases be unreliable. For instructions on how to prevent or fix this problem, see Live migration of a VM takes a long time without completing.

10.4. Verifying host CPU compatibility for virtual machine migration

For migrated virtual machines (VMs) to work correctly on the destination host, the CPUs on the source and the destination hosts must be compatible. To ensure that this is the case, calculate a common CPU baseline before you begin the migration.

Note

The instructions in this section use an example migration scenario with the following host CPUs:

Source host: Intel Core i7-8650U
Destination hosts: Intel Xeon CPU E5-2620 v2

Prerequisites

Virtualization is installed and enabled on your system.
You have administrator access to the source host and the destination host for the migration.

Procedure

On the source host, obtain its CPU features and paste them into a new XML file, such as domCaps-CPUs.xml.
```
# virsh domcapabilities | xmllint --xpath "//cpu/mode[@name='host-model']" - > domCaps-CPUs.xml
```
In the XML file, replace the <mode> </mode> tags with <cpu> </cpu>.

Optional: Verify that the content of the domCaps-CPUs.xml file looks similar to the following:

# cat domCaps-CPUs.xml

    <cpu>
          <model fallback="forbid">Skylake-Client-IBRS</model>
          <vendor>Intel</vendor>
          <feature policy="require" name="ss"/>
          <feature policy="require" name="vmx"/>
          <feature policy="require" name="pdcm"/>
          <feature policy="require" name="hypervisor"/>
          <feature policy="require" name="tsc_adjust"/>
          <feature policy="require" name="clflushopt"/>
          <feature policy="require" name="umip"/>
          <feature policy="require" name="md-clear"/>
          <feature policy="require" name="stibp"/>
          <feature policy="require" name="arch-capabilities"/>
          <feature policy="require" name="ssbd"/>
          <feature policy="require" name="xsaves"/>
          <feature policy="require" name="pdpe1gb"/>
          <feature policy="require" name="invtsc"/>
          <feature policy="require" name="ibpb"/>
          <feature policy="require" name="ibrs"/>
          <feature policy="require" name="amd-stibp"/>
          <feature policy="require" name="amd-ssbd"/>
          <feature policy="require" name="rsba"/>
          <feature policy="require" name="skip-l1dfl-vmentry"/>
          <feature policy="require" name="pschange-mc-no"/>
          <feature policy="disable" name="hle"/>
          <feature policy="disable" name="rtm"/>
    </cpu>

On the destination host, use the following command to obtain its CPU features:

# virsh domcapabilities | xmllint --xpath "//cpu/mode[@name='host-model']" -

    <mode name="host-model" supported="yes">
            <model fallback="forbid">IvyBridge-IBRS</model>
            <vendor>Intel</vendor>
            <feature policy="require" name="ss"/>
            <feature policy="require" name="vmx"/>
            <feature policy="require" name="pdcm"/>
            <feature policy="require" name="pcid"/>
            <feature policy="require" name="hypervisor"/>
            <feature policy="require" name="arat"/>
            <feature policy="require" name="tsc_adjust"/>
            <feature policy="require" name="umip"/>
            <feature policy="require" name="md-clear"/>
            <feature policy="require" name="stibp"/>
            <feature policy="require" name="arch-capabilities"/>
            <feature policy="require" name="ssbd"/>
            <feature policy="require" name="xsaveopt"/>
            <feature policy="require" name="pdpe1gb"/>
            <feature policy="require" name="invtsc"/>
            <feature policy="require" name="ibpb"/>
            <feature policy="require" name="amd-ssbd"/>
            <feature policy="require" name="skip-l1dfl-vmentry"/>
            <feature policy="require" name="pschange-mc-no"/>
    </mode>

Add the obtained CPU features from the destination host to the domCaps-CPUs.xml file on the source host. Again, replace the <mode> </mode> tags with <cpu> </cpu> and save the file.

Optional: Verify that the XML file now contains the CPU features from both hosts.

# cat domCaps-CPUs.xml

    <cpu>
          <model fallback="forbid">Skylake-Client-IBRS</model>
          <vendor>Intel</vendor>
          <feature policy="require" name="ss"/>
          <feature policy="require" name="vmx"/>
          <feature policy="require" name="pdcm"/>
          <feature policy="require" name="hypervisor"/>
          <feature policy="require" name="tsc_adjust"/>
          <feature policy="require" name="clflushopt"/>
          <feature policy="require" name="umip"/>
          <feature policy="require" name="md-clear"/>
          <feature policy="require" name="stibp"/>
          <feature policy="require" name="arch-capabilities"/>
          <feature policy="require" name="ssbd"/>
          <feature policy="require" name="xsaves"/>
          <feature policy="require" name="pdpe1gb"/>
          <feature policy="require" name="invtsc"/>
          <feature policy="require" name="ibpb"/>
          <feature policy="require" name="ibrs"/>
          <feature policy="require" name="amd-stibp"/>
          <feature policy="require" name="amd-ssbd"/>
          <feature policy="require" name="rsba"/>
          <feature policy="require" name="skip-l1dfl-vmentry"/>
          <feature policy="require" name="pschange-mc-no"/>
          <feature policy="disable" name="hle"/>
          <feature policy="disable" name="rtm"/>
    </cpu>
    <cpu>
          <model fallback="forbid">IvyBridge-IBRS</model>
          <vendor>Intel</vendor>
          <feature policy="require" name="ss"/>
          <feature policy="require" name="vmx"/>
          <feature policy="require" name="pdcm"/>
          <feature policy="require" name="pcid"/>
          <feature policy="require" name="hypervisor"/>
          <feature policy="require" name="arat"/>
          <feature policy="require" name="tsc_adjust"/>
          <feature policy="require" name="umip"/>
          <feature policy="require" name="md-clear"/>
          <feature policy="require" name="stibp"/>
          <feature policy="require" name="arch-capabilities"/>
          <feature policy="require" name="ssbd"/>
          <feature policy="require" name="xsaveopt"/>
          <feature policy="require" name="pdpe1gb"/>
          <feature policy="require" name="invtsc"/>
          <feature policy="require" name="ibpb"/>
          <feature policy="require" name="amd-ssbd"/>
          <feature policy="require" name="skip-l1dfl-vmentry"/>
          <feature policy="require" name="pschange-mc-no"/>
    </cpu>

Use the XML file to calculate the CPU feature baseline for the VM you intend to migrate.

# virsh hypervisor-cpu-baseline domCaps-CPUs.xml

    <cpu mode='custom' match='exact'>
      <model fallback='forbid'>IvyBridge-IBRS</model>
      <vendor>Intel</vendor>
      <feature policy='require' name='ss'/>
      <feature policy='require' name='vmx'/>
      <feature policy='require' name='pdcm'/>
      <feature policy='require' name='pcid'/>
      <feature policy='require' name='hypervisor'/>
      <feature policy='require' name='arat'/>
      <feature policy='require' name='tsc_adjust'/>
      <feature policy='require' name='umip'/>
      <feature policy='require' name='md-clear'/>
      <feature policy='require' name='stibp'/>
      <feature policy='require' name='arch-capabilities'/>
      <feature policy='require' name='ssbd'/>
      <feature policy='require' name='xsaveopt'/>
      <feature policy='require' name='pdpe1gb'/>
      <feature policy='require' name='invtsc'/>
      <feature policy='require' name='ibpb'/>
      <feature policy='require' name='amd-ssbd'/>
      <feature policy='require' name='skip-l1dfl-vmentry'/>
      <feature policy='require' name='pschange-mc-no'/>
    </cpu>

Open the XML configuration of the VM you intend to migrate, and replace the contents of the <cpu> section with the settings obtained in the previous step.
```
# virsh edit VM-name
```
If the VM is running, restart it.
```
# virsh reboot VM-name
```

Next steps

10.5. Sharing virtual machine disk images with other hosts

To perform a live migration of a virtual machine (VM) between supported KVM hosts, you must also migrate the storage of the running VM in a way that makes it possible for the VM to read from and write to the storage during the migration process.

One of the methods to do this is using shared VM storage. The following procedure provides instructions for sharing a locally stored VM image with the source host and the destination host by using the NFS protocol.

Prerequisites

The VM intended for migration is shut down.
Optional: A host system is available for hosting the storage that is not the source or destination host, but both the source and the destination host can reach it through the network. This is the optimal solution for shared storage and is recommended by Red Hat.
Make sure that NFS file locking is not used as it is not supported in KVM.
The NFS protocol is installed and enabled on the source and destination hosts. See
Deploying an NFS server.

Procedure

Connect to the host that will provide shared storage. In this example, it is the example-shared-storage host:

# ssh root@example-shared-storage
root@example-shared-storage's password:
Last login: Mon Sep 24 12:05:36 2019
root~#

Create a directory on the source host that will hold the disk image and will be shared with the migration hosts:
```
# mkdir /var/lib/libvirt/shared-images
```
Copy the disk image of the VM from the source host to the newly created directory. The following example copies the disk image example-disk-1 of the VM to the /var/lib/libvirt/shared-images/ directory of the example-shared-storage host:
```
# scp /var/lib/libvirt/images/example-disk-1.qcow2 root@example-shared-storage:/var/lib/libvirt/shared-images/example-disk-1.qcow2
```
On the host that you want to use for sharing the storage, add the sharing directory to the /etc/exports file. The following example shares the /var/lib/libvirt/shared-images directory with the example-source-machine and example-destination-machine hosts:
```
# /var/lib/libvirt/shared-images example-source-machine(rw,no_root_squash) example-destination-machine(rw,no\_root_squash)
```
On both the source and destination host, mount the shared directory in the /var/lib/libvirt/images directory:
```
# mount example-shared-storage:/var/lib/libvirt/shared-images /var/lib/libvirt/images
```

Verification

Start the VM on the source host and observe if it boots successfully.

Additional resources

Deploying an NFS server

10.6. Migrating a virtual machine by using the command-line interface

If the current host of a virtual machine (VM) becomes unsuitable or cannot be used anymore, or if you want to redistribute the hosting workload, you can migrate the VM to another KVM host. The following procedure provides instructions and examples for various scenarios of such migrations.

Prerequisites

The source host and the destination host both use the KVM hypervisor.
The source host and the destination host are able to reach each other over the network. Use the ping utility to verify this.
Ensure the following ports are open on the destination host.
- Port 22 is needed for connecting to the destination host by using SSH.
- Port 16509 is needed for connecting to the destination host by using TLS.
- Port 16514 is needed for connecting to the destination host by using TCP.
- Ports 49152-49215 are needed by QEMU for transfering the memory and disk migration data.
For the migration to be supportable by Red Hat, the source host and destination host must be using specific operating systems and machine types. To ensure this is the case, see Supported hosts for virtual machine migration.
The VM must be compatible with the CPU features of the destination host. To ensure this is the case, see Verifying host CPU compatibility for virtual machine migration.
The disk images of VMs that will be migrated are accessible to both the source host and the destination host. This is optional for offline migration, but required for migrating a running VM. To ensure storage accessibility for both hosts, one of the following must apply:
- You are using storage area network (SAN) logical units (LUNs).
- You are using a Ceph storage clusters.
- You have copied the disk image of the VM to the destination host, and you will use --copy-storage-all parameter when migrating the VM. Alternatively, you have created a disk image with the same format and size as the source VM disk.
- The disk image is located on a separate networked location. For instructions to set up such shared VM storage, see Sharing virtual machine disk images with other hosts.
When migrating a running VM, your network bandwidth must be higher than the rate in which the VM generates dirty memory pages.
To obtain the dirty page rate of your VM before you start the live migration, do the following:
- Monitor the rate of dirty page generation of the VM for a short period of time.
```
# virsh domdirtyrate-calc example-VM 30
```
- After the monitoring finishes, obtain its results:
```
# virsh domstats example-VM --dirtyrate
Domain: 'example-VM'
  dirtyrate.calc_status=2
  dirtyrate.calc_start_time=200942
  dirtyrate.calc_period=30
  dirtyrate.megabytes_per_second=2
```
  In this example, the VM is generating 2 MB of dirty memory pages per second. Attempting to live-migrate such a VM on a network with a bandwidth of 2 MB/s or less will cause the live migration not to progress if you do not pause the VM or lower its workload.
  To ensure that the live migration finishes successfully, Red Hat recommends that your network bandwidth is significantly greater than the VM’s dirty page generation rate.
  Note
  The value of the calc_period option might differ based on the workload and dirty page rate. You can experiment with several calc_period values to determine the most suitable period that aligns with the dirty page rate in your environment.
When migrating an existing VM in a public bridge tap network, the source and destination hosts must be located on the same network. Otherwise, the VM network will not work after migration.
Ensure that the libvirtd service is enabled and running.
```
# systemctl enable --now libvirtd.service
```

Procedure

Use the virsh migrate command with options appropriate for your migration requirements.
1. The following command migrates the example-VM-1 VM from your local host to the system connection of the example-destination host by using an SSH tunnel. The VM keeps running during the migration.
```
# virsh migrate --persistent --live example-VM-1 qemu+ssh://example-destination/system
```
2. The following commands enable you to make manual adjustments to the configuration of the example-VM-2 VM running on your local host, and then migrate the VM to the example-destination host. The migrated VM will automatically use the updated configuration.
```
# virsh dumpxml --migratable example-VM-2 > example-VM-2.xml
# vi example-VM-2.xml
# virsh migrate --live --persistent --xml example-VM-2.xml example-VM-2 qemu+ssh://example-destination/system
```
  This procedure can be useful for example when the destination host needs to use a different path to access the shared VM storage or when configuring a feature specific to the destination host.
3. The following command suspends the example-VM-3 VM from the example-source host, migrates it to the example-destination host, and instructs it to use the adjusted XML configuration, provided by the example-VM-3-alt.xml file. When the migration is completed, libvirt resumes the VM on the destination host.
```
# virsh migrate example-VM-3 qemu+ssh://example-source/system qemu+ssh://example-destination/system --xml example-VM-3-alt.xml
```
  After the migration, the VM is in the shut off state on the source host, and the migrated copy is deleted after it is shut down.

The following deletes the shut-down example-VM-4 VM from the example-source host, and moves its configuration to the example-destination host.
```
# virsh migrate --offline --persistent --undefinesource example-VM-4 qemu+ssh://example-source/system qemu+ssh://example-destination/system
```
Note that this type of migration does not require moving the VM’s disk image to shared storage. However, for the VM to be usable on the destination host, you also need to migrate the VM’s disk image. For example:
```
# scp root@example-source:/var/lib/libvirt/images/example-VM-4.qcow2 root@example-destination:/var/lib/libvirt/images/example-VM-4.qcow2
```
The following command migrates the example-VM-5 VM to the example-destination host and uses multiple parallel connections, also known as multiple file descriptors (multi-FD) migration. With multi-FD migration, you can speed up the migration by utilizing all of the available network bandwidth for the migration process.
```
# virsh migrate --parallel --parallel-connections 4 <example-VM-5> qemu+ssh://<example-destination>/system
```
This example uses 4 multi-FD channels to migrate the example-VM-5 VM. It is recommended to use one channel for each 10 Gbps of available network bandwidth. The default value is 2 channels.
1. Wait for the migration to complete. The process may take some time depending on network bandwidth, system load, and the size of the VM. If the --verbose option is not used for virsh migrate, the CLI does not display any progress indicators except errors.
  When the migration is in progress, you can use the virsh domjobinfo utility to display the migration statistics.

Verification

On the destination host, list the available VMs to verify if the VM has been migrated:
```
# virsh list
Id      Name             State
----------------------------------
10    example-VM-1      running
```
If the migration is still running, this command will list the VM state as paused.

Troubleshooting

In some cases, the target host will not be compatible with certain values of the migrated VM’s XML configuration, such as the network name or CPU type. As a result, the VM will fail to boot on the target host. To fix these problems, you can update the problematic values by using the virsh edit command. After updating the values, you must restart the VM for the changes to be applied.
If a live migration is taking a long time to complete, this may be because the VM is under heavy load and too many memory pages are changing for live migration to be possible. To fix this problem, change the migration to a non-live one by suspending the VM.
```
# virsh suspend example-VM-1
```

Additional resources

virsh migrate --help command
virsh (1) man page on your system

10.7. Live migrating a virtual machine by using the web console

If you wish to migrate a virtual machine (VM) that is performing tasks which require it to be constantly running, you can migrate that VM to another KVM host without shutting it down. This is also known as live migration. The following instructions explain how to do so by using the web console.

Warning

For tasks that modify memory pages faster than KVM can transfer them, such as heavy I/O load tasks, it is recommended that you do not live migrate the VM.

Prerequisites

You have installed the RHEL 8 web console.
For instructions, see Installing and enabling the web console.
The web console VM plug-in is installed on your system.
The source and destination hosts are running.
Ensure the following ports are open on the destination host.
- Port 22 is needed for connecting to the destination host by using SSH.
- Port 16509 is needed for connecting to the destination host by using TLS.
- Port 16514 is needed for connecting to the destination host by using TCP.
- Ports 49152-49215 are needed by QEMU for transfering the memory and disk migration data.
The VM must be compatible with the CPU features of the destination host. To ensure this is the case, see Verifying host CPU compatibility for virtual machine migration.
The VM’s disk images are located on a shared storage that is accessible to the source host as well as the destination host.
When migrating a running VM, your network bandwidth must be higher than the rate in which the VM generates dirty memory pages.
To obtain the dirty page rate of your VM before you start the live migration, do the following in your command-line interface:
1. Monitor the rate of dirty page generation of the VM for a short period of time.
```
# virsh domdirtyrate-calc vm-name 30
```
2. After the monitoring finishes, obtain its results:
```
# virsh domstats vm-name --dirtyrate
Domain: 'vm-name'
  dirtyrate.calc_status=2
  dirtyrate.calc_start_time=200942
  dirtyrate.calc_period=30
  dirtyrate.megabytes_per_second=2
```
  In this example, the VM is generating 2 MB of dirty memory pages per second. Attempting to live-migrate such a VM on a network with a bandwidth of 2 MB/s or less will cause the live migration not to progress if you do not pause the VM or lower its workload.
  To ensure that the live migration finishes successfully, Red Hat recommends that your network bandwidth is significantly greater than the VM’s dirty page generation rate.

Note

The value of the calc_period option might differ based on the workload and dirty page rate. You can experiment with several calc_period values to determine the most suitable period that aligns with the dirty page rate in your environment.

Procedure

In the Virtual Machines interface of the web console, click the Menu button ⋮ of the VM that you want to migrate.
A drop down menu appears with controls for various VM operations.
Click Migrate
The Migrate VM to another host dialog appears.
Enter the URI of the destination host.
Configure the duration of the migration:
- Permanent - Do not check the box if you wish to migrate the VM permanently. Permanent migration completely removes the VM configuration from the source host.
- Temporary - Temporary migration migrates a copy of the VM to the destination host. This copy is deleted from the destination host when the VM is shut down. The original VM remains on the source host.
Click Migrate
Your VM is migrated to the destination host.

Verification

To verify whether the VM has been successfully migrated and is working correctly:

Confirm whether the VM appears in the list of VMs available on the destination host.
Start the migrated VM and observe if it boots up.

10.8. Troubleshooting virtual machine migrations

If you are facing one of the following problems when migrating virtual machines (VMs), see the provided instructions to fix or avoid the issue.

10.8.1. Live migration of a VM takes a long time without completing

Cause

In some cases, migrating a running VM might cause the the VM to generate dirty memory pages faster than they can be migrated. When this occurs, the migration cannot complete successfully.

The following scenarios frequently cause this problem:

Live migrating a VM under a heavy load
Live migrating a VM that uses a large amount of memory, such as 1 TB or more
Important
Red Hat has successfully tested live migration of VMs with up to 6 TB of memory. However, for live migration scenarios that involve VMs with more than 1 TB of memory, customers should reach out to Red Hat technical support.

Diagnosis

If your VM live migration is taking longer than expected, use the virsh domjobinfo command to obtain the memory page data for the VM:

# virsh domjobinfo vm-name

Job type:         Unbounded
Operation:        Outgoing migration
Time elapsed:     168286974    ms
Data processed:   26.106 TiB
Data remaining:   34.383 MiB
Data total:       10.586 TiB
Memory processed: 26.106 TiB
Memory remaining: 34.383 MiB
Memory total:     10.586 TiB
Memory bandwidth: 29.056 MiB/s
Dirty rate: 17225 pages/s
Page size: 4096 bytes

In this output, the multiplication of Dirty rate and Page size is greater than Memory bandwidth. This means that the VM is generating dirty memory pages faster than the network can migrate them. As a consequence, the state of the VM on the destination host cannot converge with the state of the VM on the source host, which prevents the migration from completing.

Fix

To improve the chances that a stalled live migration finishes successfully, you can do any of the following:

Reduce the workload of the VM, especially memory updates.
- To do this, stop or cancel non-essential processes in the guest operating system of the source VM.

Increase the downtime allowed for the live migration:
1. Display the current maximum downtime at the end of a live migration for the VM that is being migrated:
```
# virsh migrate-getmaxdowntime vm-name
```
2. Set a higher maximum downtime:
```
# virsh migrate-setmaxdowntime vm-name downtime-in-miliseconds
```
  The higher you set the maximum downtime, the more likely it will be for the migration to complete.
Switch the live migration to post-copy mode.
```
# virsh migrate-start-postcopy vm-name
```
- This ensures that the memory pages of the VM can converge on the destination host, and that the migration can complete.
  However, when post-copy mode is active, the VM might slow down significantly, due to remote page requests from the destination host to the source host. In addition, if the network connection between the source host and the destination host stops working during post-copy migration, some of the VM processes may halt due to missing memory pages.
  Therefore, do not use post-copy migration if the VM availability is critical or if the migration network is unstable.
If your workload allows it, suspend the VM and let the migration finish as a non-live migration. This increases the downtime of the VM, but in most cases ensures that the migration completes successfully.

Prevention

The probability of successfully completing a live migration of a VM depends on the following:

The workload of the VM during the migration
- Before starting the migration, stop or cancel non-essential processes in the guest operating system of the VM.
The network bandwidth that the host can use for migration
- For optimal results of a live migration, the bandwidth of the network used for the migration must be significantly higher than the dirty page generation of the VM. For instructions on obtaining the VM dirty page generation rate, see the Prerequisites in Migrating a virtual machine by using the command-line interface.
- Both the source host and the destination host must have a dedicated network interface controller (NIC) for the migration. For live migrating a VM with more than 1 TB of memory, Red Hat recommends a NIC with the speed of 25 Gb/s or more.
- You can also specify the network bandwidth assigned to the live migration by using the --bandwidth option when initiating the migration. For migrating very large VMs, assign as much bandwidth as viable for your deployment.
The mode of live migration
- The default pre-copy migration mode copies memory pages repeatedly if they become dirty.
- Post-copy migration copies memory pages only once.
  To enable your live migration to switch to post-copy mode if the migration stalls, use the --postcopy option with virsh migrate when starting the migration.
The downtime specified for the deployment
- You can adjust this during the migration by using virsh migrate-setmaxdowntime as described previously.

10.9. Supported hosts for virtual machine migration

For the virtual machine (VM) migration to work properly and be supported by Red Hat, the source and destination hosts must be specific RHEL versions and machine types. The following table shows supported VM migration paths.

Table 10.2. Live migration compatibility
Migration method	Release type	Example	Support status
Forward	Major release	7.6+ 8.1	On supported RHEL 7 systems: machine types i440fx and q35
Backward	Major release	8.1 7.6+	On supported RHEL 8 systems: machine types i440fx and q35
Forward	Minor release	8.0.1+ 8.1+	On supported RHEL 7 systems: machine types i440fx and q35 on RHEL 7.6.0 and later. On supported RHEL 8 systems: machine type q35.
Backward	Minor release	8.1 8.0.1	On supported RHEL 7 systems. Fully supported for machine types i440fx and q35. On supported RHEL 8 systems: machine type q35.

Additional resources

Red Hat Enterprise Linux life cycle

10.10. Additional resources

Converting virtual machines from other hypervisors to KVM with virt-v2v in RHEL 7 and RHEL 8

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10.1. How migrating virtual machines works

10.2. Benefits of migrating virtual machines

10.3. Limitations for migrating virtual machines

10.4. Verifying host CPU compatibility for virtual machine migration

10.6. Migrating a virtual machine by using the command-line interface

10.7. Live migrating a virtual machine by using the web console

10.8. Troubleshooting virtual machine migrations

10.8.1. Live migration of a VM takes a long time without completing

10.9. Supported hosts for virtual machine migration

10.10. Additional resources

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