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docs: migration: Add a paragraph about non-shared storage migration

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Outline some of the basics and the caveats of the non-shared migration
code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
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Peter Krempa 2021-12-02 16:36:01 +01:00
parent 8841302e3d
commit 0f170de28a
1 changed files with 32 additions and 0 deletions
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docs/migration.rst
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@ -446,6 +446,38 @@ SELinux rules and settings of your system.
Supported by QEMU driver
Migration of VMs using non-shared images for disks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Libvirt by default expects that the disk images which are not explicitly network
accessed are shared between the hosts by means of a network filesystem or remote
block storage.
By default it's expected that they are in the same location, but this can be
modified by providing an updated domain XML with appropriate paths to the images
using ``--xml`` argument for ``virsh migrate``.
In case when one or more of the images are residing on local storage libvirt
can migrate them as part of the migration flow. This is enabled using
``--copy-storage-all`` flag for ``virsh migrate``. Additionally
``--migrate-disks`` parameter allows control which disks need to actually be
migrated. Without the flag all read-write disks are migrated.
On the destination the images must be either pre-created by the user having
correct format and size or alternatively if the target path resides within a
libvirt storage pool they will be automatically created.
In case when the user wishes to migrate only the topmost image from a backing
chain of images for each disks ``--copy-storage-inc`` can be used instead. User
must pre-create the images unconditionally.
In order to ensure that the migration of disks will not be overwhelmed by a
guest doing a lot of I/O to a local fast storage the
``--copy-storage-synchronous-writes`` flag ensures that newly written data is
synchronously written to the destination. This may harm I/O performance during
the migration.
.. |Migration native path| image:: migration-native.png
:class: diagram
.. |Migration tunnel path| image:: migration-tunnel.png