Migration enables an administrator to move a virtual machine instance
from one compute host to another. A typical scenario is planned
maintenance on the source host, but migration can also be useful to
redistribute the load when many VM instances are running on a specific
physical machine.

This document covers live migrations using the configuring-migrations-kvm-libvirt and VMWare
hypervisors

The migration types are:

• Non-live migration, also known as cold migration
  or simply migration.

  The instance is shut down, then moved to another hypervisor and
  restarted. The instance recognizes that it was rebooted, and the
  application running on the instance is disrupted.

  This section does not cover cold migration.

• Live migration

  The instance keeps running throughout the migration. This is useful
  when it is not possible or desirable to stop the application running on
  the instance.

  Live migrations can be classified further by the way they treat
  instance storage:

  • Shared storage-based live migration. The instance
    has ephemeral disks that are located on storage shared between the
    source and destination hosts.
  • Block live migration, or simply block migration.
    The instance has ephemeral disks that are not shared between the source
    and destination hosts. Block migration is incompatible with read-only
    devices such as CD-ROMs and Configuration Drive (config_drive).
  • Volume-backed live migration. Instances use volumes
    rather than ephemeral disks.

  Block live migration requires copying disks from the source to the
  destination host. It takes more time and puts more load on the network.
  Shared-storage and volume-backed live migration does not copy
  disks.

The following sections describe how to configure your hosts for live
migrations using the libvirt virt driver and KVM hypervisor.

Libvirt

General configuration

To enable any type of live migration, configure the compute hosts
according to the instructions below:

1. Set the following parameters in nova.conf on all
   compute hosts:

   • server_listen=0.0.0.0

     You must not make the VNC server listen to the IP address of its
     compute host, since that addresses changes when the instance is
     migrated.

     Important

     Since this setting allows VNC clients from any IP address to connect
     to instance consoles, you must take additional measures like secure
     networks or firewalls to prevent potential attackers from gaining access
     to instances.

   • instances_path must have the same value for all
     compute hosts. In this guide, the value
     /var/lib/nova/instances is assumed.

2. Ensure that name resolution on all compute hosts is identical, so
   that they can connect each other through their hostnames.

   If you use /etc/hosts for name resolution and enable
   SELinux, ensure that /etc/hosts has the correct SELinux
   context:

   # restorecon /etc/hosts

3. Enable password-less SSH so that root on one compute host can log
   on to any other compute host without providing a password. The
   libvirtd daemon, which runs as root, uses the SSH protocol
   to copy the instance to the destination and can’t know the passwords of
   all compute hosts.

   You may, for example, compile root’s public SSH keys on all compute
   hosts into an authorized_keys file and deploy that file to
   the compute hosts.

4. Configure the firewalls to allow libvirt to communicate between
   compute hosts.

   By default, libvirt uses the TCP port range from 49152 to 49261 for
   copying memory and disk contents. Compute hosts must accept connections
   in this range.

   For information about ports used by libvirt, see the libvirt
   documentation.

   Important

   Be mindful of the security risks introduced by opening ports.

Securing
live migration streams

If your compute nodes have at least libvirt 4.4.0 and QEMU 2.11.0, it
is strongly recommended to secure all your live migration streams by
taking advantage of the “QEMU-native TLS” feature. This requires a
pre-existing PKI (Public Key Infrastructure) setup. For further details
on how to set this all up, refer to the secure-live-migration-with-qemu-native-tls
document.

Block
migration, volume-based live migration

If your environment satisfies the requirements for “QEMU-native TLS”,
then block migration requires some setup; refer to the above section, Securing live migration
streams, for details. Otherwise, no additional configuration is
required for block migration and volume-backed live migration.

Be aware that block migration adds load to the network and storage
subsystems.

Shared storage

Compute hosts have many options for sharing storage, for example NFS,
shared disk array LUNs, Ceph or GlusterFS.

The next steps show how a regular Linux system might be configured as
an NFS v4 server for live migration. For detailed information and
alternative ways to configure NFS on Linux, see instructions for Ubuntu,
RHEL
and derivatives or SLES
and OpenSUSE.

1. Ensure that UID and GID of the nova user are identical on the
   compute hosts and the NFS server.

2. Create a directory with enough disk space for all instances in
   the cloud, owned by user nova. In this guide, we assume
   /var/lib/nova/instances.

3. Set the execute/search bit on the instances
   directory:

   $ chmod o+x /var/lib/nova/instances

   This allows qemu to access the instances directory
   tree.

4. Export /var/lib/nova/instances to the compute hosts.
   For example, add the following line to /etc/exports:

   /var/lib/nova/instances *(rw,sync,fsid=0,no_root_squash)

   The asterisk permits access to any NFS client. The option
   fsid=0 exports the instances directory as the NFS
   root.

After setting up the NFS server, mount the remote filesystem on all
compute hosts.

1. Assuming the NFS server’s hostname is nfs-server,
   add this line to /etc/fstab to mount the NFS root:

   nfs-server:/ /var/lib/nova/instances nfs4 defaults 0 0

2. Test NFS by mounting the instances directory and check access
   permissions for the nova user:

   $ sudo mount -a -v
   $ ls -ld /var/lib/nova/instances/
   drwxr-xr-x. 2 nova nova 6 Mar 14 21:30 /var/lib/nova/instances/

Advanced configuration for
KVM and QEMU

Live migration copies the instance’s memory from the source to the
destination compute host. After a memory page has been copied, the
instance may write to it again, so that it has to be copied again.
Instances that frequently write to different memory pages can overwhelm
the memory copy process and prevent the live migration from
completing.

This section covers configuration settings that can help live
migration of memory-intensive instances succeed.

1. Live migration completion timeout

   The Compute service will either abort or force complete a migration
   when it has been running too long. This behavior is configurable using
   the libvirt.live_migration_timeout_action
   config option. The timeout is calculated based on the instance size,
   which is the instance’s memory size in GiB. In the case of block
   migration, the size of ephemeral storage in GiB is added.

   The timeout in seconds is the instance size multiplied by the
   configurable parameter libvirt.live_migration_completion_timeout,
   whose default is 800. For example, shared-storage live migration of an
   instance with 8GiB memory will time out after 6400 seconds.

2. Instance downtime

   Near the end of the memory copy, the instance is paused for a short
   time so that the remaining few pages can be copied without interference
   from instance memory writes. The Compute service initializes this time
   to a small value that depends on the instance size, typically around 50
   milliseconds. When it notices that the memory copy does not make
   sufficient progress, it increases the time gradually.

   You can influence the instance downtime algorithm with the help of
   three configuration variables on the compute hosts:

   live_migration_downtime = 500
   live_migration_downtime_steps = 10
   live_migration_downtime_delay = 75

   live_migration_downtime sets the target maximum period
   of time Nova will try to keep the instance paused during the last part
   of the memory copy, in milliseconds. This value may be exceeded
   if there is any reduction on the transfer rate after the VM is paused.
   The default is 500.

   live_migration_downtime_steps sets the total number of
   adjustment steps until live_migration_downtime is reached.
   The default is 10 steps.

   live_migration_downtime_delay sets the time interval
   between two adjustment steps in seconds. The default is
   75.

3. Auto-convergence

   One strategy for a successful live migration of a memory-intensive
   instance is slowing the instance down. This is called auto-convergence.
   Both libvirt and QEMU implement this feature by automatically throttling
   the instance’s CPU when memory copy delays are detected.

   Auto-convergence is disabled by default. You can enable it by setting
   live_migration_permit_auto_converge=true.

   Caution

   Before enabling auto-convergence, make sure that the instance’s
   application tolerates a slow-down.

   Be aware that auto-convergence does not guarantee live migration
   success.

4. Post-copy

   Live migration of a memory-intensive instance is certain to succeed
   when you enable post-copy. This feature, implemented by libvirt and
   QEMU, activates the virtual machine on the destination host before all
   of its memory has been copied. When the virtual machine accesses a page
   that is missing on the destination host, the resulting page fault is
   resolved by copying the page from the source host.

   Post-copy is disabled by default. You can enable it by setting
   live_migration_permit_post_copy=true.

   When you enable both auto-convergence and post-copy, auto-convergence
   remains disabled.

   Caution

   The page faults introduced by post-copy can slow the instance
   down.

   When the network connection between source and destination host is
   interrupted, page faults cannot be resolved anymore and the instance is
   rebooted.

The full list of live migration configuration parameters is
documented in the Nova Configuration Options </configuration/config>

VMware

vSphere configuration

Enable vMotion on all ESX hosts which are managed by Nova by
following the instructions in this KB article.