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f6160483d9
https://bugzilla.redhat.com/show_bug.cgi?id=1322717 During offline migration, no storage is copied. Nor disks, nor NVRAM file, nor anything. We use qemu for that and because domain is not running there's nobody to copy that for us. We should document this to avoid confusing users. Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
633 lines
24 KiB
XML
633 lines
24 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Guest migration</h1>
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<ul id="toc"></ul>
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<p>
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Migration of guests between hosts is a complicated problem with many possible
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solutions, each with their own positive and negative points. For maximum
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flexibility of both hypervisor integration, and administrator deployment,
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libvirt implements several options for migration.
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</p>
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<h2><a name="transport">Network data transports</a></h2>
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<p>
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There are two options for the data transport used during migration, either
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the hypervisor's own <strong>native</strong> transport, or <strong>tunnelled</strong>
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over a libvirtd connection.
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</p>
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<h3><a name="transportnative">Hypervisor native transport</a></h3>
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<p>
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<em>Native</em> data transports may or may not support encryption, depending
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on the hypervisor in question, but will typically have the lowest computational costs
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by minimising the number of data copies involved. The native data transports will also
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require extra hypervisor-specific network configuration steps by the administrator when
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deploying a host. For some hypervisors, it might be necessary to open up a large range
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of ports on the firewall to allow multiple concurrent migration operations.
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</p>
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<p>
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<img class="diagram" src="migration-native.png" alt="Migration native path"/>
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</p>
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<h3><a name="transporttunnel">libvirt tunnelled transport</a></h3>
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<p>
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<em>Tunnelled</em> data transports will always be capable of strong encryption
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since they are able to leverage the capabilities built in to the libvirt RPC protocol.
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The downside of a tunnelled transport, however, is that there will be extra data copies
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involved on both the source and destinations hosts as the data is moved between libvirtd
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and the hypervisor. This is likely to be a more significant problem for guests with
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very large RAM sizes, which dirty memory pages quickly. On the deployment side, tunnelled
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transports do not require any extra network configuration over and above what's already
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required for general libvirtd <a href="remote.html">remote access</a>, and there is only
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need for a single port to be open on the firewall to support multiple concurrent
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migration operations.
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</p>
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<p>
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<img class="diagram" src="migration-tunnel.png" alt="Migration tunnel path"/>
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</p>
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<h2><a name="flow">Communication control paths/flows</a></h2>
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<p>
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Migration of virtual machines requires close co-ordination of the two
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hosts involved, as well as the application invoking the migration,
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which may be on the source, the destination, or a third host.
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</p>
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<h3><a name="flowmanageddirect">Managed direct migration</a></h3>
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<p>
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With <em>managed direct</em> migration, the libvirt client process
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controls the various phases of migration. The client application must
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be able to connect and authenticate with the libvirtd daemons on both
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the source and destination hosts. There is no need for the two libvirtd
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daemons to communicate with each other. If the client application
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crashes, or otherwise loses its connection to libvirtd during the
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migration process, an attempt will be made to abort the migration and
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restart the guest CPUs on the source host. There may be scenarios
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where this cannot be safely done, in which cases the guest will be
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left paused on one or both of the hosts.
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</p>
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<p>
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<img class="diagram" src="migration-managed-direct.png" alt="Migration direct, managed"/>
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</p>
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<h3><a name="flowpeer2peer">Managed peer to peer migration</a></h3>
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<p>
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With <em>peer to peer</em> migration, the libvirt client process only
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talks to the libvirtd daemon on the source host. The source libvirtd
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daemon controls the entire migration process itself, by directly
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connecting the destination host libvirtd. If the client application crashes,
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or otherwise loses its connection to libvirtd, the migration process
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will continue uninterrupted until completion. Note that the
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source libvirtd uses its own credentials (typically root) to
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connect to the destination, rather than the credentials used
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by the client to connect to the source; if these differ, it is
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common to run into a situation where a client can connect to the
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destination directly but the source cannot make the connection to
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set up the peer-to-peer migration.
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</p>
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<p>
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<img class="diagram" src="migration-managed-p2p.png" alt="Migration peer-to-peer"/>
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</p>
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<h3><a name="flowunmanageddirect">Unmanaged direct migration</a></h3>
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<p>
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With <em>unmanaged direct</em> migration, neither the libvirt client
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or libvirtd daemon control the migration process. Control is instead
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delegated to the hypervisor's over management services (if any). The
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libvirt client merely initiates the migration via the hypervisor's
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management layer. If the libvirt client or libvirtd crash, the
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migration process will continue uninterrupted until completion.
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</p>
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<p>
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<img class="diagram" src="migration-unmanaged-direct.png" alt="Migration direct, unmanaged"/>
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</p>
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<h2><a name="security">Data security</a></h2>
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<p>
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Since the migration data stream includes a complete copy of the guest
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OS RAM, snooping of the migration data stream may allow compromise
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of sensitive guest information. If the virtualization hosts have
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multiple network interfaces, or if the network switches support
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tagged VLANs, then it is very desirable to separate guest network
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traffic from migration or management traffic.
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</p>
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<p>
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In some scenarios, even a separate network for migration data may
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not offer sufficient security. In this case it is possible to apply
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encryption to the migration data stream. If the hypervisor does not
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itself offer encryption, then the libvirt tunnelled migration
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facility should be used.
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</p>
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<h2><a name="offline">Offline migration</a></h2>
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<p>
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Offline migration transfers inactive the definition of a domain
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(which may or may not be active). After successful completion, the
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domain remains in its current state on the source host and is defined
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but inactive on the destination host. It's a bit more clever than
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<code>virsh dumpxml</code> on source host followed by
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<code>virsh define</code> on destination host, as offline migration
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will run the pre-migration hook to update the domain XML on
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destination host. Currently, copying non-shared storage or other file
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based storages (e.g. UEFI variable storage) is not supported during
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offline migration.
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</p>
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<h2><a name="uris">Migration URIs</a></h2>
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<p>
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Initiating a guest migration requires the client application to
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specify up to three URIs, depending on the choice of control
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flow and/or APIs used. The first URI is that of the libvirt
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connection to the source host, where the virtual guest is
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currently running. The second URI is that of the libvirt
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connection to the destination host, where the virtual guest
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will be moved to (and in peer-to-peer migrations, this is from
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the perspective of the source, not the client). The third URI is
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a hypervisor specific
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URI used to control how the guest will be migrated. With
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any managed migration flow, the first and second URIs are
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compulsory, while the third URI is optional. With the
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unmanaged direct migration mode, the first and third URIs are
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compulsory and the second URI is not used.
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</p>
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<p>
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Ordinarily management applications only need to care about the
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first and second URIs, which are both in the normal libvirt
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connection URI format. Libvirt will then automatically determine
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the hypervisor specific URI, by looking up the target host's
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configured hostname. There are a few scenarios where the management
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application may wish to have direct control over the third URI.
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</p>
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<ol>
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<li>The configured hostname is incorrect, or DNS is broken. If a
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host has a hostname which will not resolve to match one of its
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public IP addresses, then libvirt will generate an incorrect
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URI. In this case the management application should specify the
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hypervisor specific URI explicitly, using an IP address, or a
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correct hostname.</li>
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<li>The host has multiple network interfaces. If a host has multiple
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network interfaces, it might be desirable for the migration data
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stream to be sent over a specific interface for either security
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or performance reasons. In this case the management application
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should specify the hypervisor specific URI, using an IP address
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associated with the network to be used.</li>
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<li>The firewall restricts what ports are available. When libvirt
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generates a migration URI it will pick a port number using hypervisor
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specific rules. Some hypervisors only require a single port to be
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open in the firewalls, while others require a whole range of port
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numbers. In the latter case the management application may wish
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to choose a specific port number outside the default range in order
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to comply with local firewall policies.</li>
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</ol>
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<h2><a name="config">Configuration file handling</a></h2>
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<p>
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There are two types of virtual machine known to libvirt. A <em>transient</em>
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guest only exists while it is running, and has no configuration file stored
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on disk. A <em>persistent</em> guest maintains a configuration file on disk
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even when it is not running.
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</p>
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<p>
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By default, a migration operation will not attempt to change any configuration
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files that may be stored on either the source or destination host. It is the
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administrator, or management application's, responsibility to manage distribution
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of configuration files (if desired). It is important to note that the <code>/etc/libvirt</code>
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directory <strong>MUST NEVER BE SHARED BETWEEN HOSTS</strong>. There are some
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typical scenarios that might be applicable:
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</p>
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<ul>
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<li>Centralized configuration files outside libvirt, in shared storage. A cluster
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aware management application may maintain all the master guest configuration
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files in a cluster filesystem. When attempting to start a guest, the config
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will be read from the cluster FS and used to deploy a persistent guest.
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For migration the configuration will need to be copied to the destination
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host and removed on the original.
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</li>
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<li>Centralized configuration files outside libvirt, in a database. A data center
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management application may not storage configuration files at all. Instead it
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may generate libvirt XML on the fly when a guest is booted. It will typically
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use transient guests, and thus not have to consider configuration files during
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migration.
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</li>
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<li>Distributed configuration inside libvirt. The configuration file for each
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guest is copied to every host where the guest is able to run. Upon migration
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the existing config merely needs to be updated with any changes
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</li>
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<li>Ad-hoc configuration management inside libvirt. Each guest is tied to a
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specific host and rarely migrated. When migration is required, the config
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is moved from one host to the other.
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</li>
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</ul>
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<p>
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As mentioned above, libvirt will not touch configuration files during
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migration by default. The <code>virsh</code> command has two flags to
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influence this behaviour. The <code>--undefine-source</code> flag
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will cause the configuration file to be removed on the source host
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after a successful migration. The <code>--persist</code> flag will
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cause a configuration file to be created on the destination host
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after a successful migration. The following table summarizes the
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configuration file handling in all possible state and flag
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combinations.
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</p>
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<table class="data">
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<thead>
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<tr class="head">
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<th colspan="3">Before migration</th>
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<th colspan="2">Flags</th>
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<th colspan="3">After migration</th>
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</tr>
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<tr class="subhead">
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<th>Guest type</th>
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<th>Source config</th>
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<th>Dest config</th>
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<th>--undefine-source</th>
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<th>--persist</th>
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<th>Guest type</th>
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<th>Source config</th>
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<th>Dest config</th>
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</tr>
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</thead>
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<tbody>
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<!-- src:N, dst:N -->
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<!-- src:N, dst:Y -->
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<!-- src:Y dst:N -->
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td>Transient</td>
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<td class="n">N</td>
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<td class="n">N</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<!-- src:Y dst:Y -->
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="n">N</td>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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</tr>
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<tr>
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<td>Persistent</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td class="y">Y</td>
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<td>Persistent</td>
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<td class="n">N</td>
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<td class="y">Y</td>
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</tr>
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</tbody>
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</table>
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<h2><a name="scenarios">Migration scenarios</a></h2>
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|
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<h3><a name="scenarionativedirect">Native migration, client to two libvirtd servers</a></h3>
|
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|
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<p>
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At an API level this requires use of virDomainMigrate, without the
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VIR_MIGRATE_PEER2PEER flag set. The destination libvirtd server
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will automatically determine the native hypervisor URI for migration
|
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based off the primary hostname. To force migration over an alternate
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network interface the optional hypervisor specific URI must be provided
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</p>
|
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|
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<pre>
|
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syntax: virsh migrate GUESTNAME DEST-LIBVIRT-URI [HV-URI]
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eg using default network interface
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virsh migrate web1 qemu+ssh://desthost/system
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virsh migrate web1 xen+tls://desthost/system
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eg using secondary network interface
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virsh migrate web1 qemu://desthost/system tcp://10.0.0.1/
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virsh migrate web1 xen+tcp://desthost/system xenmigr:10.0.0.1/
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</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 & 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 & 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>
|
|
|
|
</body>
|
|
</html>
|