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 administrator deployment, libvirt implements several options for migration.
There are two options for the data transport used during migration, either the hypervisor's own native transport, or tunnelled over a libvirtd connection.
Native 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.
Tunnelled 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 remote access, and there is only need for a single port to be open on the firewall to support multiple concurrent migration operations.
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.
With managed direct 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.
With peer to peer 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.
With unmanaged direct 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.
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.
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.
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
virsh dumpxml on source host followed by
virsh define 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 or other file
based storages (e.g. UEFI variable storage) is not supported during
offline migration.
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.
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.
There are two types of virtual machines known to libvirt. A transient guest only exists while it is running, and has no configuration file stored on disk. A persistent guest maintains a configuration file on disk even when it is not running.
By default, a migration operation will not attempt to modify 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 /etc/libvirt
directory MUST NEVER BE SHARED BETWEEN HOSTS. There are some
typical scenarios that might be applicable:
As mentioned above, libvirt will not modify configuration files during
migration by default. The virsh command has two flags to
influence this behaviour. The --undefine-source flag
will cause the configuration file to be removed on the source host
after a successful migration. The --persist 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.
| Before migration | Flags | After migration | |||||
|---|---|---|---|---|---|---|---|
| Source type | Source config | Dest config | --undefine-source | --persist | Dest type | Source config | Dest config |
| Transient | N | N | N | N | Transient | N | N |
| Transient | N | N | Y | N | Transient | N | N |
| Transient | N | N | N | Y | Persistent | N | Y |
| Transient | N | N | Y | Y | Persistent | N | Y |
| Transient | N | Y | N | N | Persistent | N | Y (unchanged dest config) |
| Transient | N | Y | Y | N | Persistent | N | Y (unchanged dest config) |
| Transient | N | Y | N | Y | Persistent | N | Y (replaced with source) |
| Transient | N | Y | Y | Y | Persistent | N | Y (replaced with source) |
| Persistent | Y | N | N | N | Transient | Y | N |
| Persistent | Y | N | Y | N | Transient | N | N |
| Persistent | Y | N | N | Y | Persistent | Y | Y |
| Persistent | Y | N | Y | Y | Persistent | N | Y |
| Persistent | Y | Y | N | N | Persistent | Y | Y (unchanged dest config) |
| Persistent | Y | Y | Y | N | Persistent | N | Y (unchanged dest config) |
| Persistent | Y | Y | N | Y | Persistent | Y | Y (replaced with source) |
| Persistent | Y | Y | Y | Y | Persistent | N | Y (replaced with source) |
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
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/
Supported by Xen, QEMU, VMware and VirtualBox drivers
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.
This mode cannot be invoked from virsh
Supported by QEMU driver
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.
This mode cannot be invoked from virsh
Supported by QEMU driver
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.
syntax: virsh migrate GUESTNAME HV-URI
eg using same libvirt URI for all connections
virsh migrate --direct web1 xenmigr://desthost/
Supported by Xen driver
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).
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
Supported by the QEMU driver
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).
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
Supported by QEMU driver