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openstack-manuals/doc/admin-guide-cloud/source/compute-configuring-migrations.rst

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.. _section_configuring-compute-migrations:
====================
Configure migrations
====================
.. :ref:`_configuring-migrations-kvm-libvirt`
.. :ref:`_configuring-migrations-xenserver`
.. note::
Only cloud administrators can perform live migrations. If your cloud
is configured to use cells, you can perform live migration within
but not between cells.
Migration enables an administrator to move a virtual-machine instance
from one compute host to another. This feature is useful when a compute
host requires maintenance. Migration can also be useful to redistribute
the load when many VM instances are running on a specific physical
machine.
The migration types are:
- **Non-live migration** (sometimes referred to simply as 'migration').
The instance is shut down for a period of time to be moved to another
hypervisor. In this case, the instance recognizes that it was
rebooted.
- **Live migration** (or 'true live migration'). Almost no instance
downtime. Useful when the instances must be kept running during the
migration. The different types of live migration are:
- **Shared storage-based live migration**. Both hypervisors have
access to shared storage.
- **Block live migration**. No shared storage is required.
Incompatible with read-only devices such as CD-ROMs and
`Configuration Drive (config\_drive) <http://docs.openstack.org/user-guide/cli_config_drive.html>`_.
- **Volume-backed live migration**. Instances are backed by volumes
rather than ephemeral disk, no shared storage is required, and
migration is supported (currently only available for libvirt-based
hypervisors).
The following sections describe how to configure your hosts and compute
nodes for migrations by using the KVM and XenServer hypervisors.
.. _configuring-migrations-kvm-libvirt:
KVM-Libvirt
~~~~~~~~~~~
.. :ref:`_configuring-migrations-kvm-shared-storage`
.. :ref:`_configuring-migrations-kvm-block-migration`
.. _configuring-migrations-kvm-shared-storage:
Shared storage
--------------
.. :ref:`_section_example-compute-install`
.. :ref:`_true-live-migration-kvm-libvirt`
**Prerequisites**
- **Hypervisor:** KVM with libvirt
- **Shared storage:** :file:`NOVA-INST-DIR/instances/` (for example,
:file:`/var/lib/nova/instances`) has to be mounted by shared storage.
This guide uses NFS but other options, including the
`OpenStack Gluster Connector <http://gluster.org/community/documentation//index.php/OSConnect>`_
are available.
- **Instances:** Instance can be migrated with iSCSI-based volumes.
**Notes**
- Because the Compute service does not use the libvirt live
migration functionality by default, guests are suspended before
migration and might experience several minutes of downtime. For
details, see `Enabling true live migration`.
- Compute calculates the amount of downtime required using the RAM size of
the disk being migrated, in accordance with the ``live_migration_downtime``
configuration parameters. Migration downtime is measured in steps, with an
exponential backoff between each step. This means that the maximum
downtime between each step starts off small, and is increased in ever
larger amounts as Compute waits for the migration to complete. This gives
the guest a chance to complete the migration successfully, with a minimum
amount of downtime.
- This guide assumes the default value for ``instances_path`` in
your :file:`nova.conf` file (:file:`NOVA-INST-DIR/instances`). If you
have changed the ``state_path`` or ``instances_path`` variables,
modify the commands accordingly.
- You must specify ``vncserver_listen=0.0.0.0`` or live migration
will not work correctly.
- You must specify the ``instances_path`` in each node that runs
nova-compute. The mount point for ``instances_path`` must be the
same value for each node, or live migration will not work
correctly.
.. _section_example-compute-install:
Example Compute installation environment
----------------------------------------
- Prepare at least three servers. In this example, we refer to the
servers as ``HostA``, ``HostB``, and ``HostC``:
- ``HostA`` is the Cloud Controller, and should run these services:
nova-api, nova-scheduler, ``nova-network``, cinder-volume, and
``nova-objectstore``.
- ``HostB`` and ``HostC`` are the compute nodes that run
nova-compute.
Ensure that ``NOVA-INST-DIR`` (set with ``state_path`` in the
:file:`nova.conf` file) is the same on all hosts.
- In this example, ``HostA`` is the NFSv4 server that exports
``NOVA-INST-DIR/instances`` directory. ``HostB`` and ``HostC`` are
NFSv4 clients that mount ``HostA``.
**Configuring your system**
#. Configure your DNS or ``/etc/hosts`` and ensure it is consistent across
all hosts. Make sure that the three hosts can perform name resolution
with each other. As a test, use the :command:`ping` command to ping each host
from one another:
.. code:: console
$ ping HostA
$ ping HostB
$ ping HostC
#. Ensure that the UID and GID of your Compute and libvirt users are
identical between each of your servers. This ensures that the
permissions on the NFS mount works correctly.
#. Ensure you can access SSH without a password and without
StrictHostKeyChecking between ``HostB`` and ``HostC`` as ``nova``
user (set with the owner of nova-compute service). Direct access
from one compute host to another is needed to copy the VM file
across. It is also needed to detect if the source and target
compute nodes share a storage subsystem.
#. Export ``NOVA-INST-DIR/instances`` from ``HostA``, and ensure it is
readable and writable by the Compute user on ``HostB`` and ``HostC``.
For more information, see: `SettingUpNFSHowTo <https://help.ubuntu.com/community/SettingUpNFSHowTo>`_
or `CentOS/Red Hat: Setup NFS v4.0 File Server <http://www.cyberciti.biz/faq/centos-fedora-rhel-nfs-v4-configuration/>`_
#. Configure the NFS server at ``HostA`` by adding the following line to
the :file:`/etc/exports` file:
.. code:: ini
NOVA-INST-DIR/instances HostA/255.255.0.0(rw,sync,fsid=0,no_root_squash)
Change the subnet mask (``255.255.0.0``) to the appropriate value to
include the IP addresses of ``HostB`` and ``HostC``. Then restart the
NFS server:
.. code:: console
# /etc/init.d/nfs-kernel-server restart
# /etc/init.d/idmapd restart
#. On both compute nodes, enable the 'execute/search' bit on your shared
directory to allow qemu to be able to use the images within the
directories. On all hosts, run the following command:
.. code:: console
$ chmod o+x NOVA-INST-DIR/instances
#. Configure NFS on ``HostB`` and ``HostC`` by adding the following line to
the :file:`/etc/fstab` file
.. code:: console
HostA:/ /NOVA-INST-DIR/instances nfs4 defaults 0 0
Ensure that you can mount the exported directory
.. code:: console
$ mount -a -v
Check that ``HostA`` can see the :file:`NOVA-INST-DIR/instances/`
directory
.. code:: console
$ ls -ld NOVA-INST-DIR/instances/
drwxr-xr-x 2 nova nova 4096 2012-05-19 14:34 nova-install-dir/instances/
Perform the same check on ``HostB`` and ``HostC``, paying special
attention to the permissions (Compute should be able to write)
.. code-block:: console
:linenos:
$ ls -ld NOVA-INST-DIR/instances/
drwxr-xr-x 2 nova nova 4096 2012-05-07 14:34 nova-install-dir/instances/
$ df -k
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 921514972 4180880 870523828 1% /
none 16498340 1228 16497112 1% /dev
none 16502856 0 16502856 0% /dev/shm
none 16502856 368 16502488 1% /var/run
none 16502856 0 16502856 0% /var/lock
none 16502856 0 16502856 0% /lib/init/rw
HostA: 921515008 101921792 772783104 12% /var/lib/nova/instances ( <--- this line is important.)
#. Update the libvirt configurations so that the calls can be made
securely. These methods enable remote access over TCP and are not
documented here.
- SSH tunnel to libvirtd's UNIX socket
- libvirtd TCP socket, with GSSAPI/Kerberos for auth+data encryption
- libvirtd TCP socket, with TLS for encryption and x509 client certs
for authentication
- libvirtd TCP socket, with TLS for encryption and Kerberos for
authentication
Restart libvirt. After you run the command, ensure that libvirt is
successfully restarted
.. code:: console
# stop libvirt-bin && start libvirt-bin
$ ps -ef | grep libvirt
root 1145 1 0 Nov27 ? 00:00:03 /usr/sbin/libvirtd -d -l\
#. Configure your firewall to allow libvirt to communicate between nodes.
By default, libvirt listens on TCP port 16509, and an ephemeral TCP
range from 49152 to 49261 is used for the KVM communications. Based on
the secure remote access TCP configuration you chose, be careful which
ports you open, and always understand who has access. For information
about ports that are used with libvirt,
see the `libvirt documentation <http://libvirt.org/remote.html#Remote_libvirtd_configuration>`_.
#. Configure the downtime required for the migration by adjusting these
parameters in the :file:`nova.conf` file:
.. code:: ini
live_migration_downtime
live_migration_downtime_steps
live_migration_downtime_delay
The ``live_migration_downtime`` parameter sets the maximum permitted
downtime for a live migration, in milliseconds. This setting defaults to
500 milliseconds.
The ``live_migration_downtime_steps`` parameter sets the total number of
incremental steps to reach the maximum downtime value. This setting
defaults to 10 steps.
The ``live_migration_downtime_delay`` parameter sets the amount of time
to wait between each step, in seconds. This setting defaults to 75 seconds.
#. You can now configure other options for live migration. In most cases, you
will not need to configure any options. For advanced configuration options,
see the `OpenStack Configuration Reference Guide <http://docs.openstack.org/
kilo/config-reference/content/list-of-compute-config-options.html
#config_table_nova_livemigration>`_.
.. _true-live-migration-kvm-libvirt:
Enabling true live migration
----------------------------
Prior to the Kilo release, the Compute service did not use the libvirt
live migration function by default. To enable this function, add the
following line to the ``[libvirt]`` section of the :file:`nova.conf` file:
.. code:: ini
live_migration_flag=VIR_MIGRATE_UNDEFINE_SOURCE,VIR_MIGRATE_PEER2PEER,VIR_MIGRATE_LIVE,VIR_MIGRATE_TUNNELLED
On versions older than Kilo, the Compute service does not use libvirt's
live migration by default because there is a risk that the migration
process will never end. This can happen if the guest operating system
uses blocks on the disk faster than they can be migrated.
.. _configuring-migrations-kvm-block-migration:
Block migration
---------------
Configuring KVM for block migration is exactly the same as the above
configuration in :ref:`configuring-migrations-kvm-shared-storage`
the section called shared storage, except that ``NOVA-INST-DIR/instances``
is local to each host rather than shared. No NFS client or server
configuration is required.
.. note::
- To use block migration, you must use the :option:`--block-migrate`
parameter with the live migration command.
- Block migration is incompatible with read-only devices such as
CD-ROMs and `Configuration Drive (config_drive) <http://docs.openstack.org/user-guide/cli_config_drive.html>`_.
- Since the ephemeral drives are copied over the network in block
migration, migrations of instances with heavy I/O loads may never
complete if the drives are writing faster than the data can be
copied over the network.
.. _configuring-migrations-xenserver:
XenServer
~~~~~~~~~
.. :ref:Shared Storage
.. :ref:Block migration
.. _configuring-migrations-xenserver-shared-storage:
Shared storage
--------------
**Prerequisites**
- **Compatible XenServer hypervisors**. For more information, see the
`Requirements for Creating Resource Pools <http://docs.vmd.citrix.com/XenServer/6.0.0/1.0/en_gb/reference.html#pooling_homogeneity_requirements>`_ section of the XenServer
Administrator's Guide.
- **Shared storage**. An NFS export, visible to all XenServer hosts.
.. note::
For the supported NFS versions, see the
`NFS VHD <http://docs.vmd.citrix.com/XenServer/6.0.0/1.0/en_gb/reference.html#id1002701>`_
section of the XenServer Administrator's Guide.
To use shared storage live migration with XenServer hypervisors, the
hosts must be joined to a XenServer pool. To create that pool, a host
aggregate must be created with specific metadata. This metadata is used
by the XAPI plug-ins to establish the pool.
**Using shared storage live migrations with XenServer Hypervisors**
#. Add an NFS VHD storage to your master XenServer, and set it as the
default storage repository. For more information, see NFS VHD in the
XenServer Administrator's Guide.
#. Configure all compute nodes to use the default storage repository
(``sr``) for pool operations. Add this line to your :file:`nova.conf`
configuration files on all compute nodes:
.. code:: ini
sr_matching_filter=default-sr:true
#. Create a host aggregate. This command creates the aggregate, and then
displays a table that contains the ID of the new aggregate
.. code:: console
$ nova aggregate-create POOL_NAME AVAILABILITY_ZONE
Add metadata to the aggregate, to mark it as a hypervisor pool
.. code:: console
$ nova aggregate-set-metadata AGGREGATE_ID hypervisor_pool=true
$ nova aggregate-set-metadata AGGREGATE_ID operational_state=created
Make the first compute node part of that aggregate
.. code:: console
$ nova aggregate-add-host AGGREGATE_ID MASTER_COMPUTE_NAME
The host is now part of a XenServer pool.
#. Add hosts to the pool
.. code:: console
$ nova aggregate-add-host AGGREGATE_ID COMPUTE_HOST_NAME
.. note::
The added compute node and the host will shut down to join the host
to the XenServer pool. The operation will fail if any server other
than the compute node is running or suspended on the host.
.. _configuring-migrations-xenserver-block-migration:
Block migration
---------------
- **Compatible XenServer hypervisors**.
The hypervisors must support the Storage XenMotion feature.
See your XenServer manual to make sure your edition
has this feature.
.. note::
- To use block migration, you must use the :option:`--block-migrate`
parameter with the live migration command.
- Block migration works only with EXT local storage storage
repositories, and the server must not have any volumes attached.
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