Chapter 10. Migrating virtual machines 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. 10.1. How migrating virtual machines works You can migrate a running virtual machine (VM) without interrupting the workload, with only a small downtime, by using a live migration. By default, the migrated VM is transient on the destination host, and remains defined also on the source host. The essential part of a live migration is transferring the state of the VM's memory and of any attached virtualized devices to a destination host. For the VM to remain functional on the destination host, the VM's disk images must remain available to it. To migrate a shut-off VM, you must use an offline migration, which copies the VM's configuration to the destination host. 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, for VMs that modify memory pages faster than KVM can transfer them, such as VMs under heavy I/O load, the live migration might fail. (1) The VM's disk images must be accessible both to the source host and the destination host during the migration. (2) 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 accessible to the source or destination host during migration, and can be copied or moved manually to the destination host instead. (1) For possible solutions, see: Additional virsh migrate options for live migrations (2) 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 Note For easier management of large-scale migrations, explore other Red Hat products, such as: OpenShift Virtualization Red Hat OpenStack Platform Additional resources Benefits of migrating virtual machines Sharing virtual machine disk images with other hosts 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 . 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. Both the source and destination hosts use specific RHEL versions that are supported for VM migration, see Supported hosts for virtual machine migration The physical 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 . Physical machine firmware versions and settings 10.4. Migrating a virtual machine by using the command line 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. You can perform a live migration or an offline migration . For differences between the two scenarios, see How migrating virtual machines works . Prerequisites Hypervisor: The source host and the destination host both use the KVM hypervisor. Network connection: The source host and the destination host are able to reach each other over the network. Use the ping utility to verify this. Open ports: 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 transferring the memory and disk migration data. Hosts: 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 . CPU: 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 . Storage: 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 created a disk image with the same format and size as the source VM disk and you will use the --copy-storage-all parameter when migrating the VM. 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 . Network bandwidth: 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. After the monitoring finishes, obtain its results: 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. Bridge tap network specifics: 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. libvirtd: Ensure that the libvirtd service is enabled and running. Procedure To migrate a VM from one host to another, use the virsh migrate command. Offline migration The following command migrates a shut-off example-VM VM from your local host to the system connection of the example-destination host by using an SSH tunnel. Live migration The following command migrates the example-VM 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. Wait for the migration to complete. The process might 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. Multi-FD live migration You can use multiple parallel connections to the destination host during the live migration. This is 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. This example uses 4 multi-FD channels to migrate the <example_VM> VM. It is recommended to use one channel for each 10 Gbps of available network bandwidth. The default value is 2 channels. Live migration with an increased downtime limit To improve the reliability of a live migration, you can set the maxdowntime parameter, which specifies the maximum amount of time, in milliseconds, the VM can be paused during live migration. Setting a larger downtime can help to ensure the migration completes successfully. Post-copy migration If your VM has a large memory footprint, you can perform a post-copy migration, which transfers the source VM's CPU state first and immediately starts the migrated VM on the destination host. The source VM's memory pages are transferred after the migrated VM is already running on the destination host. Because of this, a post-copy migration can result in a smaller downtime of the migrated VM. However, the running VM on the destination host might try to access memory pages that have not yet been transferred, which causes a page fault . If too many page faults occur during the migration, the performance of the migrated VM can be severely degraded. Given the potential complications of a post-copy migration, it is recommended to use the following command that starts a standard live migration and switches to a post-copy migration if the live migration cannot be finished in a specified amount of time. Auto-converged live migration If your VM is under a heavy memory workload, you can use the --auto-converge option. This option automatically slows down the execution speed of the VM's CPU. As a consequence, this CPU throttling can help to slow down memory writes, which means the live migration might succeed even in VMs with a heavy memory workload. However, the CPU throttling does not help to resolve workloads where memory writes are not directly related to CPU execution speed, and it can negatively impact the performance of the VM during a live migration. Verification For offline migration: On the destination host, list the available VMs to verify that the VM was migrated successfully. For live migration: On the destination host, list the available VMs to verify the state of the destination VM: If the state of the VM is listed as running , it means that the migration is finished. However, if the live migration is still in progress, the state of the destination VM will be listed as paused . For post-copy migration: On the source host, list the available VMs to verify the state of the source VM. On the destination host, list the available VMs to verify the state of the destination VM. If the state of the source VM is listed as shut off and the state of the destination VM is listed as running , it means that the migration is finished. Additional resources virsh migrate --help command virsh (1) man page on your system 10.5. Live migrating a virtual machine by using the web console If you want 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. Prerequisites You have installed the RHEL 8 web console. You have enabled the cockpit service. Your user account is allowed to log in to the web console. For instructions, see Installing and enabling the web console . The web console VM plugin is installed on your system . Hypervisor: The source host and the destination host both use the KVM hypervisor. Hosts: The source and destination hosts are running. Open ports: 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. CPU: 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 . Storage: 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 created a disk image with the same format and size as the source VM disk and you will use the --copy-storage-all parameter when migrating the VM. 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 . Network bandwidth: 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 on the command line: Monitor the rate of dirty page generation of the VM for a short period of time. After the monitoring finishes, obtain its results: 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. Bridge tap network specifics: 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. 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 want 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. 10.6. 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 . The virt_use_nfs SELinux boolean is set to on . Procedure Connect to the host that will provide shared storage. In this example, it is the example-shared-storage host: Create a directory on the example-shared-storage host that will hold the disk image and that will be shared with the migration hosts: 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: 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: Run the exportfs -a command for the changes in the /etc/exports file to take effect. On both the source and destination host, mount the shared directory in the /var/lib/libvirt/images directory: Verification Start the VM on the source host and observe if it boots successfully. Additional resources Deploying an NFS server 10.7. 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 . 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: On the destination host, use the following command to obtain its CPU features: 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. Use the XML file to calculate the CPU feature baseline for the VM you intend to migrate. 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 step. If the VM is running, shut down the VM and start it again. steps Sharing virtual machine disk images with other hosts Migrating a virtual machine by using the command line Live-migrating a virtual machine by using the web console 10.8. 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 10.9. Additional resources Converting virtual machines from other hypervisors to KVM with virt-v2v in RHEL 7 and RHEL 8

Commands:
virsh domdirtyrate-calc <example_VM> 30
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
systemctl enable --now libvirtd.service
virsh migrate --offline --persistent <example_VM> qemu+ssh:// example-destination /system
virsh migrate --live --persistent <example_VM> qemu+ssh:// example-destination /system
virsh migrate --live --persistent --parallel --parallel-connections 4 <example_VM> qemu+ssh:// <example-destination> /system
virsh migrate-setmaxdowntime <example_VM> <time_interval_in_milliseconds>
virsh migrate --live --persistent --postcopy --timeout <time_interval_in_seconds> --timeout-postcopy <example_VM> qemu+ssh:// <example-destination> /system
virsh migrate --live --persistent --auto-converge <example_VM> qemu+ssh:// <example-destination> /system
virsh list --all Id Name State ---------------------------------- 10 example-VM-1 shut off
virsh list --all Id Name State ---------------------------------- 10 example-VM-1 running
virsh list --all Id Name State ---------------------------------- 10 example-VM-1 shut off
virsh list --all Id Name State ---------------------------------- 10 example-VM-1 running
virsh domdirtyrate-calc vm-name 30
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
setsebool virt_use_nfs 1
ssh root@ example-shared-storage root@example-shared-storage's password: Last login: Mon Sep 24 12:05:36 2019 root~#
mkdir /var/lib/libvirt/shared-images
scp /var/lib/libvirt/images/ example-disk-1 .qcow2 root@ example-shared-storage :/var/lib/libvirt/shared-images/ example-disk-1 .qcow2
/var/lib/libvirt/shared-images example-source-machine (rw,no_root_squash) example-destination-machine (rw,no\_root_squash)
exportfs -a
mount example-shared-storage :/var/lib/libvirt/shared-images /var/lib/libvirt/images
virsh domcapabilities | xmllint --xpath "//cpu/mode[@name='host-model']" - > domCaps-CPUs.xml
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>
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>
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>
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>
virsh edit <vm_name>
virsh shutdown <vm_name> virsh start <vm_name>