Chapter 4. KVM Live Migration

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This chapter covers migrating guest virtual machines running on one host physical machine to another. In both instances, the host physical machines are running the KVM hypervisor.
Migration describes the process of moving a guest virtual machine from one host physical machine to another. This is possible because guest virtual machines are running in a virtualized environment instead of directly on the hardware. Migration is useful for:
  • Load balancing - guest virtual machines can be moved to host physical machines with lower usage when their host physical machine becomes overloaded, or another host physical machine is under-utilized.
  • Hardware independence - when we need to upgrade, add, or remove hardware devices on the host physical machine, we can safely relocate guest virtual machines to other host physical machines. This means that guest virtual machines do not experience any downtime for hardware improvements.
  • Energy saving - guest virtual machines can be redistributed to other host physical machines and can thus be powered off to save energy and cut costs in low usage periods.
  • Geographic migration - guest virtual machines can be moved to another location for lower latency or in serious circumstances.
Migration works by sending the state of the guest virtual machine's memory and any virtualized devices to a destination host physical machine. It is recommended to use shared, networked storage to store the guest virtual machine's images to be migrated. It is also recommended to use libvirt-managed storage pools for shared storage when migrating virtual machines.
Migrations can be performed live or not.
In a live migration, the guest virtual machine continues to run on the source host physical machine while its memory pages are transferred, in order, to the destination host physical machine. During migration, KVM monitors the source for any changes in pages it has already transferred, and begins to transfer these changes when all of the initial pages have been transferred. KVM also estimates transfer speed during migration, so when the remaining amount of data to transfer will take a certain configurable period of time (10 milliseconds by default), KVM suspends the original guest virtual machine, transfers the remaining data, and resumes the same guest virtual machine on the destination host physical machine.
A migration that is not performed live, suspends the guest virtual machine, then moves an image of the guest virtual machine's memory to the destination host physical machine. The guest virtual machine is then resumed on the destination host physical machine and the memory the guest virtual machine used on the source host physical machine is freed. The time it takes to complete such a migration depends on network bandwidth and latency. If the network is experiencing heavy use or low bandwidth, the migration will take much longer.
If the original guest virtual machine modifies pages faster than KVM can transfer them to the destination host physical machine, offline migration must be used, as live migration would never complete.

4.1. Live Migration Requirements

Migrating guest virtual machines requires the following:

Migration requirements

  • A guest virtual machine installed on shared storage using one of the following protocols:
    • Fibre Channel-based LUNs
    • iSCSI
    • FCoE
    • NFS
    • GFS2
    • SCSI RDMA protocols (SCSI RCP): the block export protocol used in Infiniband and 10GbE iWARP adapters
  • The migration platforms and versions should be checked against table Table 4.1, “Live Migration Compatibility”. It should also be noted that Red Hat Enterprise Linux 6 supports live migration of guest virtual machines using raw and qcow2 images on shared storage.
  • Both systems must have the appropriate TCP/IP ports open. In cases where a firewall is used, refer to the Red Hat Enterprise Linux Virtualization Security Guide which can be found at for detailed port information.
  • A separate system exporting the shared storage medium. Storage should not reside on either of the two host physical machines being used for migration.
  • Shared storage must mount at the same location on source and destination systems. The mounted directory names must be identical. Although it is possible to keep the images using different paths, it is not recommended. Note that, if you are intending to use virt-manager to perform the migration, the path names must be identical. If however you intend to use virsh to perform the migration, different network configurations and mount directories can be used with the help of --xml option or pre-hooks when doing migrations. Even without shared storage, migration can still succeed with the option --copy-storage-all (deprecated). For more information on prehooks, refer to, and for more information on the XML option, refer to Chapter 20, Manipulating the Domain XML.
  • When migration is attempted on an existing guest virtual machine in a public bridge+tap network, the source and destination host physical machines must be located in the same network. Otherwise, the guest virtual machine network will not operate after migration.
  • In Red Hat Enterprise Linux 5 and 6, the default cache mode of KVM guest virtual machines is set to none, which prevents inconsistent disk states. Setting the cache option to none (using virsh attach-disk cache none, for example), causes all of the guest virtual machine's files to be opened using the O_DIRECT flag (when calling the open syscall), thus bypassing the host physical machine's cache, and only providing caching on the guest virtual machine. Setting the cache mode to none prevents any potential inconsistency problems, and when used makes it possible to live-migrate virtual machines. For information on setting cache to none, refer to Section 13.3, “Adding Storage Devices to Guests”.
Make sure that the libvirtd service is enabled (# chkconfig libvirtd on) and running (# service libvirtd start). It is also important to note that the ability to migrate effectively is dependent on the parameter settings in the /etc/libvirt/libvirtd.conf configuration file.

Procedure 4.1. Configuring libvirtd.conf

  1. Opening the libvirtd.conf requires running the command as root:
    # vim /etc/libvirt/libvirtd.conf
  2. Change the parameters as needed and save the file.
  3. Restart the libvirtd service:
    # service libvirtd restart
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