Configure live migrations

Configure live migrations

Migration enables an administrator to move a virtual machine instance from one compute host to another. A typical scenario is planned maintenance on the source host, but migration can also be useful to redistribute the load when many VM instances are running on a specific physical machine.

This document covers live migrations using the KVM-libvirt and XenServer hypervisors.


Not all Compute service hypervisor drivers support live-migration, or support all live-migration features.

Consult the Hypervisor Support Matrix to determine which hypervisors support live-migration.

See the Hypervisor configuration pages for details on hypervisor-specific configuration settings.

The migration types are:

  • Non-live migration, also known as cold migration or simply migration.

    The instance is shut down, then moved to another hypervisor and restarted. The instance recognizes that it was rebooted, and the application running on the instance is disrupted.

    This section does not cover cold migration.

  • Live migration

    The instance keeps running throughout the migration. This is useful when it is not possible or desirable to stop the application running on the instance.

    Live migrations can be classified further by the way they treat instance storage:

    • Shared storage-based live migration. The instance has ephemeral disks that are located on storage shared between the source and destination hosts.
    • Block live migration, or simply block migration. The instance has ephemeral disks that are not shared between the source and destination hosts. Block migration is incompatible with read-only devices such as CD-ROMs and Configuration Drive (config_drive).
    • Volume-backed live migration. Instances use volumes rather than ephemeral disks.

    Block live migration requires copying disks from the source to the destination host. It takes more time and puts more load on the network. Shared-storage and volume-backed live migration does not copy disks.


In a multi-cell cloud, instances can be live migrated to a different host in the same cell, but not across cells.

The following sections describe how to configure your hosts for live migrations using the KVM and XenServer hypervisors.


General configuration

To enable any type of live migration, configure the compute hosts according to the instructions below:

  1. Set the following parameters in nova.conf on all compute hosts:

    • vncserver_listen=

      You must not make the VNC server listen to the IP address of its compute host, since that addresses changes when the instance is migrated.


      Since this setting allows VNC clients from any IP address to connect to instance consoles, you must take additional measures like secure networks or firewalls to prevent potential attackers from gaining access to instances.

    • instances_path must have the same value for all compute hosts. In this guide, the value /var/lib/nova/instances is assumed.

  2. Ensure that name resolution on all compute hosts is identical, so that they can connect each other through their hostnames.

    If you use /etc/hosts for name resolution and enable SELinux, ensure that /etc/hosts has the correct SELinux context:

    # restorecon /etc/hosts
  3. Enable password-less SSH so that root on one compute host can log on to any other compute host without providing a password. The libvirtd daemon, which runs as root, uses the SSH protocol to copy the instance to the destination and can't know the passwords of all compute hosts.

    You may, for example, compile root's public SSH keys on all compute hosts into an authorized_keys file and deploy that file to the compute hosts.

  4. Configure the firewalls to allow libvirt to communicate between compute hosts.

    By default, libvirt uses the TCP port range from 49152 to 49261 for copying memory and disk contents. Compute hosts must accept connections in this range.

    For information about ports used by libvirt, see the libvirt documentation.


    Be mindful of the security risks introduced by opening ports.

Block migration, volume-based live migration

No additional configuration is required for block migration and volume-backed live migration.

Be aware that block migration adds load to the network and storage subsystems.

Shared storage

Compute hosts have many options for sharing storage, for example NFS, shared disk array LUNs, Ceph or GlusterFS.

The next steps show how a regular Linux system might be configured as an NFS v4 server for live migration. For detailed information and alternative ways to configure NFS on Linux, see instructions for Ubuntu, RHEL and derivatives or SLES and OpenSUSE.

  1. Ensure that UID and GID of the nova user are identical on the compute hosts and the NFS server.

  2. Create a directory with enough disk space for all instances in the cloud, owned by user nova. In this guide, we assume /var/lib/nova/instances.

  3. Set the execute/search bit on the instances directory:

    $ chmod o+x /var/lib/nova/instances

    This allows qemu to access the instances directory tree.

  4. Export /var/lib/nova/instances to the compute hosts. For example, add the following line to /etc/exports:

    /var/lib/nova/instances *(rw,sync,fsid=0,no_root_squash)

    The asterisk permits access to any NFS client. The option fsid=0 exports the instances directory as the NFS root.

After setting up the NFS server, mount the remote filesystem on all compute hosts.

  1. Assuming the NFS server's hostname is nfs-server, add this line to /etc/fstab to mount the NFS root:

    nfs-server:/ /var/lib/nova/instances nfs4 defaults 0 0
  2. Test NFS by mounting the instances directory and check access permissions for the nova user:

    $ sudo mount -a -v
    $ ls -ld /var/lib/nova/instances/
    drwxr-xr-x. 2 nova nova 6 Mar 14 21:30 /var/lib/nova/instances/

Advanced configuration for KVM and QEMU

Live migration copies the instance's memory from the source to the destination compute host. After a memory page has been copied, the instance may write to it again, so that it has to be copied again. Instances that frequently write to different memory pages can overwhelm the memory copy process and prevent the live migration from completing.

This section covers configuration settings that can help live migration of memory-intensive instances succeed.

  1. Live migration completion timeout

    The Compute service aborts a migration when it has been running for too long. The timeout is calculated based on the instance size, which is the instance's memory size in GiB. In the case of block migration, the size of ephemeral storage in GiB is added.

    The timeout in seconds is the instance size multiplied by the configurable parameter live_migration_completion_timeout, whose default is 800. For example, shared-storage live migration of an instance with 8GiB memory will time out after 6400 seconds.

  2. Live migration progress timeout

    The Compute service also aborts a live migration when it detects that memory copy is not making progress for a certain time. You can set this time, in seconds, through the configurable parameter live_migration_progress_timeout.

    In Ocata, the default value of live_migration_progress_timeout is 0, which disables progress timeouts. You should not change this value, since the algorithm that detects memory copy progress has been determined to be unreliable. It may be re-enabled in future releases.

  3. Instance downtime

    Near the end of the memory copy, the instance is paused for a short time so that the remaining few pages can be copied without interference from instance memory writes. The Compute service initializes this time to a small value that depends on the instance size, typically around 50 milliseconds. When it notices that the memory copy does not make sufficient progress, it increases the time gradually.

    You can influence the instance downtime algorithm with the help of three configuration variables on the compute hosts:

    live_migration_downtime = 500
    live_migration_downtime_steps = 10
    live_migration_downtime_delay = 75

    live_migration_downtime sets the maximum permitted downtime for a live migration, in milliseconds. The default is 500.

    live_migration_downtime_steps sets the total number of adjustment steps until live_migration_downtime is reached. The default is 10 steps.

    live_migration_downtime_delay sets the time interval between two adjustment steps in seconds. The default is 75.

  4. Auto-convergence

    One strategy for a successful live migration of a memory-intensive instance is slowing the instance down. This is called auto-convergence. Both libvirt and QEMU implement this feature by automatically throttling the instance's CPU when memory copy delays are detected.

    Auto-convergence is disabled by default. You can enable it by setting live_migration_permit_auto_converge=true.


    Before enabling auto-convergence, make sure that the instance's application tolerates a slow-down.

    Be aware that auto-convergence does not guarantee live migration success.

  5. Post-copy

    Live migration of a memory-intensive instance is certain to succeed when you enable post-copy. This feature, implemented by libvirt and QEMU, activates the virtual machine on the destination host before all of its memory has been copied. When the virtual machine accesses a page that is missing on the destination host, the resulting page fault is resolved by copying the page from the source host.

    Post-copy is disabled by default. You can enable it by setting live_migration_permit_post_copy=true.

    When you enable both auto-convergence and post-copy, auto-convergence remains disabled.


    The page faults introduced by post-copy can slow the instance down.

    When the network connection between source and destination host is interrupted, page faults cannot be resolved anymore and the instance is rebooted.

The full list of live migration configuration parameters is documented in the Nova Configuration Options


Shared storage


  • Compatible XenServer hypervisors.

    For more information, see the Requirements for Creating Resource Pools section of the XenServer Administrator's Guide.

  • Shared storage.

    An NFS export, visible to all XenServer hosts.


    For the supported NFS versions, see the NFS VHD section of the XenServer Administrator's Guide.

To use shared storage live migration with XenServer hypervisors, the hosts must be joined to a XenServer pool. To create that pool, a host aggregate must be created with specific metadata. This metadata is used by the XAPI plug-ins to establish the pool.

Using shared storage live migrations with XenServer Hypervisors

  1. Add an NFS VHD storage to your master XenServer, and set it as the default storage repository. For more information, see NFS VHD in the XenServer Administrator's Guide.

  2. Configure all compute nodes to use the default storage repository (sr) for pool operations. Add this line to your nova.conf configuration files on all compute nodes:

  3. Create a host aggregate. This command creates the aggregate, and then displays a table that contains the ID of the new aggregate

    $ openstack aggregate create --zone AVAILABILITY_ZONE POOL_NAME

    Add metadata to the aggregate, to mark it as a hypervisor pool

    $ openstack aggregate set --property hypervisor_pool=true AGGREGATE_ID
    $ openstack aggregate set --property operational_state=created AGGREGATE_ID

    Make the first compute node part of that aggregate

    $ openstack aggregate add host AGGREGATE_ID MASTER_COMPUTE_NAME

    The host is now part of a XenServer pool.

  4. Add hosts to the pool

    $ openstack aggregate add host AGGREGATE_ID COMPUTE_HOST_NAME


    The added compute node and the host will shut down to join the host to the XenServer pool. The operation will fail if any server other than the compute node is running or suspended on the host.

Block migration

  • Compatible XenServer hypervisors.

    The hypervisors must support the Storage XenMotion feature. See your XenServer manual to make sure your edition has this feature.


    • To use block migration, you must use the --block-migrate parameter with the live migration command.
    • Block migration works only with EXT local storage storage repositories, and the server must not have any volumes attached.
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