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6d4bca5b1f
Use of the wrong attribute name caused the table of contents to
be useless. Fix suggested by Daniel P. Berrange.
* docs/migration.html.in: Use correct anchoring attribute.
(cherry picked from commit eeb8c924ce)
616 lines
23 KiB
HTML
616 lines
23 KiB
HTML
<html>
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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 adminsitrator 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="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 interaces. 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 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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<h3><a name="scenarionativedirect">Native migration, client to two libvirtd servers</a></h3>
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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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<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>
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<p>
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Supported by Xen, QEMU, VMWare and VirtualBox drivers
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</p>
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<h3><a name="scenarionativepeer2peer">Native migration, client to and peer2peer between, two libvirtd servers</a></h3>
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<p>
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virDomainMigrate, with the VIR_MIGRATE_PEER2PEER flag set,
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using the libvirt URI format for the 'uri' parameter. The
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destination libvirtd server will automatically determine
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the native hypervisor URI for migration, based off the
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primary hostname. The optional uri parameter controls how
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the source libvirtd connects to the destination libvirtd,
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in case it is not accessible using the same address that
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the client uses to connect to the destination, or a different
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encryption/auth scheme is required. There is no
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scope for forcing an alternative network interface for the
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native migration data with this method.
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</p>
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<p>
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This mode cannot be invoked from virsh
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</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>
|