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    <h1>Guest migration</h1>

    <ul id="toc"></ul>

    <p>
      Migration of guests between hosts is a complicated problem with many possible
      solutions, each with their own positive and negative points. For maximum
      flexibility of both hypervisor integration, and adminsitrator deployment,
      libvirt implements several options for migration.
    </p>

    <h2><a name="transport">Network data transports</a></h2>

    <p>
      There are two options for the data transport used during migration, either
      the hypervisor's own <strong>native</strong> transport, or <strong>tunnelled</strong>
      over a libvirtd connection.
    </p>

    <h3><a name="transportnative">Hypervisor native transport</a></h3>
    <p>
      <em>Native</em> data transports may or may not support encryption, depending
      on the hypervisor in question, but will typically have the lowest computational costs
      by minimising the number of data copies involved. The native data transports will also
      require extra hypervisor-specific network configuration steps by the administrator when
      deploying a host. For some hypervisors, it might be necessary to open up a large range
      of ports on the firewall to allow multiple concurrent migration operations.
    </p>

    <p>
      <img class="diagram" src="migration-native.png" alt="Migration native path"/>
    </p>

    <h3><a name="transporttunnel">libvirt tunnelled transport</a></h3>
    <p>
      <em>Tunnelled</em> data transports will always be capable of strong encryption
      since they are able to leverage the capabilities built in to the libvirt RPC protocol.
      The downside of a tunnelled transport, however, is that there will be extra data copies
      involved on both the source and destinations hosts as the data is moved between libvirtd
      and the hypervisor. This is likely to be a more significant problem for guests with
      very large RAM sizes, which dirty memory pages quickly. On the deployment side, tunnelled
      transports do not require any extra network configuration over and above what's already
      required for general libvirtd <a href="remote.html">remote access</a>, and there is only
      need for a single port to be open on the firewall to support multiple concurrent
      migration operations.
    </p>

    <p>
      <img class="diagram" src="migration-tunnel.png" alt="Migration tunnel path"/>
    </p>

    <h2><a name="flow">Communication control paths/flows</a></h2>

    <p>
      Migration of virtual machines requires close co-ordination of the two
      hosts involved, as well as the application invoking the migration,
      which may be on the source, the destination, or a third host.
    </p>

    <h3><a name="flowmanageddirect">Managed direct migration</a></h3>

    <p>
      With <em>managed direct</em> migration, the libvirt client process
      controls the various phases of migration. The client application must
      be able to connect and authenticate with the libvirtd daemons on both
      the source and destination hosts. There is no need for the two libvirtd
      daemons to communicate with each other. If the client application
      crashes, or otherwise loses its connection to libvirtd during the
      migration process, an attempt will be made to abort the migration and
      restart the guest CPUs on the source host. There may be scenarios
      where this cannot be safely done, in which cases the guest will be
      left paused on one or both of the hosts.
    </p>

    <p>
      <img class="diagram" src="migration-managed-direct.png" alt="Migration direct, managed"/>
    </p>


    <h3><a name="flowpeer2peer">Managed peer to peer migration</a></h3>

    <p>
      With <em>peer to peer</em> migration, the libvirt client process only
      talks to the libvirtd daemon on the source host. The source libvirtd
      daemon controls the entire migration process itself, by directly
      connecting the destination host libvirtd. If the client application crashes,
      or otherwise loses its connection to libvirtd, the migration process
      will continue uninterrupted until completion.  Note that the
      source libvirtd uses its own credentials (typically root) to
      connect to the destination, rather than the credentials used
      by the client to connect to the source; if these differ, it is
      common to run into a situation where a client can connect to the
      destination directly but the source cannot make the connection to
      set up the peer-to-peer migration.
    </p>

    <p>
      <img class="diagram" src="migration-managed-p2p.png" alt="Migration peer-to-peer"/>
    </p>


    <h3><a name="flowunmanageddirect">Unmanaged direct migration</a></h3>

    <p>
      With <em>unmanaged direct</em> migration, neither the libvirt client
      or libvirtd daemon control the migration process. Control is instead
      delegated to the hypervisor's over management services (if any). The
      libvirt client merely initiates the migration via the hypervisor's
      management layer. If the libvirt client or libvirtd crash, the
      migration process will continue uninterrupted until completion.
    </p>

    <p>
      <img class="diagram" src="migration-unmanaged-direct.png" alt="Migration direct, unmanaged"/>
    </p>


    <h2><a name="security">Data security</a></h2>

    <p>
      Since the migration data stream includes a complete copy of the guest
      OS RAM, snooping of the migration data stream may allow compromise
      of sensitive guest information. If the virtualization hosts have
      multiple network interfaces, or if the network switches support
      tagged VLANs, then it is very desirable to separate guest network
      traffic from migration or management traffic.
    </p>

    <p>
      In some scenarios, even a separate network for migration data may
      not offer sufficient security. In this case it is possible to apply
      encryption to the migration data stream. If the hypervisor does not
      itself offer encryption, then the libvirt tunnelled migration
      facility should be used.
    </p>

    <h2><a name="offline">Offline migration</a></h2>

    <p>
      Offline migration transfers inactive the definition of a domain
      (which may or may not be active). After successful completion, the
      domain remains in its current state on the source host and is defined
      but inactive on the destination host. It's a bit more clever than
      <code>virsh dumpxml</code> on source host followed by
      <code>virsh define</code> on destination host, as offline migration
      will run the pre-migration hook to update the domain XML on
      destination host. Currently, copying non-shared storage is not
      supported during offline migration.
    </p>

    <h2><a name="uris">Migration URIs</a></h2>

    <p>
      Initiating a guest migration requires the client application to
      specify up to three URIs, depending on the choice of control
      flow and/or APIs used. The first URI is that of the libvirt
      connection to the source host, where the virtual guest is
      currently running. The second URI is that of the libvirt
      connection to the destination host, where the virtual guest
      will be moved to (and in peer-to-peer migrations, this is from
      the perspective of the source, not the client). The third URI is
      a hypervisor specific
      URI used to control how the guest will be migrated. With
      any managed migration flow, the first and second URIs are
      compulsory, while the third URI is optional. With the
      unmanaged direct migration mode, the first and third URIs are
      compulsory and the second URI is not used.
    </p>

    <p>
      Ordinarily management applications only need to care about the
      first and second URIs, which are both in the normal libvirt
      connection URI format. Libvirt will then automatically determine
      the hypervisor specific URI, by looking up the target host's
      configured hostname. There are a few scenarios where the management
      application may wish to have direct control over the third URI.
    </p>

    <ol>
      <li>The configured hostname is incorrect, or DNS is broken. If a
        host has a hostname which will not resolve to match one of its
        public IP addresses, then libvirt will generate an incorrect
        URI. In this case the management application should specify the
        hypervisor specific URI explicitly, using an IP address, or a
        correct hostname.</li>
      <li>The host has multiple network interaces. If a host has multiple
        network interfaces, it might be desirable for the migration data
        stream to be sent over a specific interface for either security
        or performance reasons. In this case the management application
        should specify the hypervisor specific URI, using an IP address
        associated with the network to be used.</li>
      <li>The firewall restricts what ports are available. When libvirt
        generates a migration URI it will pick a port number using hypervisor
        specific rules. Some hypervisors only require a single port to be
        open in the firewalls, while others require a whole range of port
        numbers. In the latter case the management application may wish
        to choose a specific port number outside the default range in order
        to comply with local firewall policies.</li>
    </ol>

    <h2><a name="config">Configuration file handling</a></h2>

    <p>
      There are two types of virtual machine known to libvirt. A <em>transient</em>
      guest only exists while it is running, and has no configuration file stored
      on disk. A <em>persistent</em> guest maintains a configuration file on disk
      even when it is not running.
    </p>

    <p>
      By default, a migration operation will not attempt to change any configuration
      files that may be stored on either the source or destination host. It is the
      administrator, or management application's, responsibility to manage distribution
      of configuration files (if desired). It is important to note that the <code>/etc/libvirt</code>
      directory <strong>MUST NEVER BE SHARED BETWEEN HOSTS</strong>. There are some
      typical scenarios that might be applicable:
    </p>

    <ul>
      <li>Centralized configuration files outside libvirt, in shared storage. A cluster
        aware  management application may maintain all the master guest configuration
        files in a cluster filesystem. When attempting to start a guest, the config
        will be read from the cluster FS and used to deploy a persistent guest.
        For migration the configuration will need to be copied to the destination
        host and removed on the original.
      </li>
      <li>Centralized configuration files outside libvirt, in a database. A data center
        management application may not storage configuration files at all. Instead it
        may generate libvirt XML on the fly when a guest is booted. It will typically
        use transient guests, and thus not have to consider configuration files during
        migration.
      </li>
      <li>Distributed configuration inside libvirt. The configuration file for each
        guest is copied to every host where the guest is able to run. Upon migration
        the existing config merely needs to be updated with any changes
      </li>
      <li>Ad-hoc configuration management inside libvirt. Each guest is tied to a
        specific host and rarely migrated. When migration is required, the config
        is moved from one host to the other.
      </li>
    </ul>

    <p>
      As mentioned above, libvirt will not touch configuration files during
      migration by default. The <code>virsh</code> command has two flags to
      influence this behaviour. The  <code>--undefine-source</code> flag
      will cause the configuration file to be removed on the source host
      after a successful migration. The <code>--persist</code> flag will
      cause a configuration file to be created on the destination host
      after a successful migration. The following table summarizes the
      configuration file handling in all possible state and flag
      combinations.
    </p>

    <table class="data">
      <thead>
        <tr class="head">
          <th colspan="3">Before migration</th>
          <th colspan="2">Flags</th>
          <th colspan="3">After migration</th>
        </tr>
        <tr class="subhead">
          <th>Guest type</th>
          <th>Source config</th>
          <th>Dest config</th>
          <th>--undefine-source</th>
          <th>--persist</th>
          <th>Guest type</th>
          <th>Source config</th>
          <th>Dest config</th>
        </tr>
      </thead>
      <tbody>
        <!-- src:N, dst:N -->
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>

        <!-- src:N, dst:Y -->
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>

        <!-- src:Y dst:N -->
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="y">Y</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td>Transient</td>
          <td class="n">N</td>
          <td class="n">N</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>

        <!-- src:Y dst:Y -->
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="n">N</td>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="n">N</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
        </tr>
        <tr>
          <td>Persistent</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td class="y">Y</td>
          <td>Persistent</td>
          <td class="n">N</td>
          <td class="y">Y</td>
        </tr>
      </tbody>
    </table>

    <h2><a name="scenarios">Migration scenarios</a></h2>


    <h3><a name="scenarionativedirect">Native migration, client to two libvirtd servers</a></h3>

    <p>
      At an API level this requires use of virDomainMigrate, without the
      VIR_MIGRATE_PEER2PEER flag set. The destination libvirtd server
      will automatically determine the native hypervisor URI for migration
      based off the primary hostname. To force migration over an alternate
      network interface the optional hypervisor specific URI must be provided
    </p>

    <pre>
      syntax: virsh migrate GUESTNAME DEST-LIBVIRT-URI [HV-URI]


      eg using default network interface

      virsh migrate web1 qemu+ssh://desthost/system
      virsh migrate web1 xen+tls://desthost/system


      eg using secondary network interface

      virsh migrate web1 qemu://desthost/system tcp://10.0.0.1/
      virsh migrate web1 xen+tcp://desthost/system  xenmigr:10.0.0.1/
    </pre>

    <p>
      Supported by Xen, QEMU, VMWare and VirtualBox drivers
    </p>

    <h3><a name="scenarionativepeer2peer">Native migration, client to and peer2peer between, two libvirtd servers</a></h3>

    <p>
      virDomainMigrate, with the VIR_MIGRATE_PEER2PEER flag set,
      using the libvirt URI format for the 'uri' parameter. The
      destination libvirtd server will automatically determine
      the native hypervisor URI for migration, based off the
      primary hostname. The optional uri parameter controls how
      the source libvirtd connects to the destination libvirtd,
      in case it is not accessible using the same address that
      the client uses to connect to the destination, or a different
      encryption/auth scheme is required. There is no
      scope for forcing an alternative network interface for the
      native migration data with this method.
    </p>

    <p>
      This mode cannot be invoked from virsh
    </p>

    <p>
      Supported by QEMU driver
    </p>

    <h3><a name="scenariotunnelpeer2peer1">Tunnelled migration, client and peer2peer between two libvirtd servers</a></h3>

    <p>
      virDomainMigrate, with the VIR_MIGRATE_PEER2PEER &amp; VIR_MIGRATE_TUNNELLED
      flags set, using the libvirt URI format for the 'uri' parameter. The
      destination libvirtd server will automatically determine
      the native hypervisor URI for migration, based off the
      primary hostname. The optional uri parameter controls how
      the source libvirtd connects to the destination libvirtd,
      in case it is not accessible using the same address that
      the client uses to connect to the destination, or a different
      encryption/auth scheme is required. The native hypervisor URI
      format is not used at all.
    </p>

    <p>
      This mode cannot be invoked from virsh
    </p>

    <p>
      Supported by QEMU driver
    </p>

    <h3><a name="nativedirectunmanaged">Native migration, client to one libvirtd server</a></h3>

    <p>
      virDomainMigrateToURI, without the VIR_MIGRATE_PEER2PEER flag set,
      using a hypervisor specific URI format for the 'uri' parameter.
      There is no use or requirement for a destination libvirtd instance
      at all. This is typically used when the hypervisor has its own
      native management daemon available to handle incoming migration
      attempts on the destination.
    </p>

    <pre>
      syntax: virsh migrate GUESTNAME HV-URI


      eg using same libvirt URI for all connections

      virsh migrate --direct web1 xenmigr://desthost/
    </pre>

    <p>
      Supported by Xen driver
    </p>

    <h3><a name="nativepeer2peer">Native migration, peer2peer between two libvirtd servers</a></h3>

    <p>
      virDomainMigrateToURI, with the VIR_MIGRATE_PEER2PEER flag set,
      using the libvirt URI format for the 'uri' parameter. The
      destination libvirtd server will automatically determine
      the native hypervisor URI for migration, based off the
      primary hostname. There is no scope for forcing an alternative
      network interface for the native migration data with this
      method.  The destination URI must be reachable using the source
      libvirtd credentials (which are not necessarily the same as the
      credentials of the client in connecting to the source).
    </p>

    <pre>
      syntax: virsh migrate GUESTNAME DEST-LIBVIRT-URI [ALT-DEST-LIBVIRT-URI]


      eg using same libvirt URI for all connections

      virsh migrate --p2p web1 qemu+ssh://desthost/system


      eg using different libvirt URI auth scheme for peer2peer connections

      virsh migrate --p2p web1 qemu+ssh://desthost/system qemu+tls:/desthost/system


      eg using different libvirt URI hostname for peer2peer connections

      virsh migrate --p2p web1 qemu+ssh://desthost/system qemu+ssh://10.0.0.1/system
    </pre>

    <p>
      Supported by the QEMU driver
    </p>

    <h3><a name="scenariotunnelpeer2peer2">Tunnelled migration, peer2peer between two libvirtd servers</a></h3>

    <p>
      virDomainMigrateToURI, with the VIR_MIGRATE_PEER2PEER &amp; VIR_MIGRATE_TUNNELLED
      flags set, using the libvirt URI format for the 'uri' parameter. The
      destination libvirtd server will automatically determine
      the native hypervisor URI for migration, based off the
      primary hostname. The optional uri parameter controls how
      the source libvirtd connects to the destination libvirtd,
      in case it is not accessible using the same address that
      the client uses to connect to the destination, or a different
      encryption/auth scheme is required. The native hypervisor URI
      format is not used at all.  The destination URI must be
      reachable using the source libvirtd credentials (which are not
      necessarily the same as the credentials of the client in
      connecting to the source).
    </p>

    <pre>
      syntax: virsh migrate GUESTNAME DEST-LIBVIRT-URI [ALT-DEST-LIBVIRT-URI]


      eg using same libvirt URI for all connections

      virsh migrate --p2p --tunnelled web1 qemu+ssh://desthost/system


      eg using different libvirt URI auth scheme for peer2peer connections

      virsh migrate --p2p --tunnelled web1 qemu+ssh://desthost/system qemu+tls:/desthost/system


      eg using different libvirt URI hostname for peer2peer connections

      virsh migrate --p2p --tunnelled web1 qemu+ssh://desthost/system qemu+ssh://10.0.0.1/system
    </pre>

    <p>
      Supported by QEMU driver
    </p>

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