Microsoft Live Migration – A High Level Overview

1.0 Introduction

Live Migration is arguably the most eagerly anticipated new feature of Microsoft Windows Server 2008 R2 Hyper-V and Hyper-V Server 2008 R2. This builds on, and is in addition to, the Quick Migration feature first introduced in Windows Server 2008 Hyper-V and Hyper-V Server 2008.

It gives Microsoft a direct equivalent to its competitor’s features, specifically VMware VMotion and Citrix XenMotion; indeed, under the covers, all three employ a very similar means of operation. 

2.0 Live Migration

2.1 Description

While Quick Migration enables the moving of a running Virtual Machine (VM) from one Hyper-V host to another, a disadvantage of the “save, move and restore the VM” technique it uses is that the VM is unavailable for a period of time (around 10 to 60 seconds) and hence service is interrupted.

Live Migration takes a completely different approach, enabling the same VM-relocation capability but with the benefit of no perceived downtime. Why perceived? Because while the Live Migration process requires a period of downtime (for the transfer of execution of the VM between the hosts), it is negligible and typically less than the TCP timeout, so no service interruption is apparent to remote users.

2.2 Benefits

This key feature of Live Migration offers numerous agility, cost and productivity benefits, including:

• VMs can be migrated to a specific host at any time in order to optimise VM performance, cluster balancing or consolidation ratios, without negatively impacting users.
• A host requiring maintenance can have the VMs migrated from it, the work performed and be put back into service during normal business hours, without interrupting service.
• Power savings can be made during periods of lower processing demand by migrating VMs from a host and powering it off until the extra capacity is required again.

2.3 Requirements

The requirements for Live Migration are similar to those for Quick Migration and are set out below:

• Two or more Hyper-V hosts running Windows Server 2008 R2 x64 (Enterprise or Datacenter edition) or Hyper-V Server 2008 R2
• Each host uses the same make and model of processor (A feature called Processor Compatibility Mode gives a degree of flexibility but is beyond the scope of this article. More information is available in the document referenced in the Useful Links section below).
• Each host is configured for Microsoft Failover Clustering
• Each host has access to the same networks and shared storage
• The cluster is configured with a dedicated network for Live Migration traffic

Additionally, it is strongly recommended to:

• Create a Clustered Shared Volume (CSV) for the storage of VMs (CSV is also a new feature of Microsoft Windows Server 2008 R2 Hyper-V and Hyper-V Server 2008 R2).
• Dedicate a 1 Gigabit Ethernet connection on each host for Live Migration traffic

2.4 Implementation

This is beyond the scope of this article. Detailed Live Migration implementation steps are available in the document referenced in the Useful Links section below.

2.5 Initiation

The Live Migration of a VM can be initiated in three ways:

• Via the Failover Cluster Management console
• Via the Virtual Machine Manager administration console
• Via a Windows Management Instrumentation (WMI) or PowerShell script

Note: The maximum supported number of simultaneous Live Migrations is limited to half the number of hosts in the cluster.

2.6 Operation

When the Live Migration of a VM has been initiated, Hyper-V takes the following actions:

• Preparation:
  The source host establishes a network connection to the destination host and defines a “skeleton” VM on it with an identical configuration to the VM to be Live Migrated.
• Transfer:
  All the memory pages for the VM are transferred from the source to the destination. Any source memory pages changed during this process are tracked and, when the first transfer is complete, these are sent in a second pass. This process is repeated until the number of changed pages to send is below a certain threshold and can be transferred in negligible amount of time.
• Handover:
  A final memory transfer pass is made before control of the VMs storage and responsibility for its execution is transferred from the source to the destination. The VM has been Live Migrated and is running on the destination host.
• Cleanup:
  A network broadcast is issued which forces the physical switch to learn the new location of the VM and the now redundant VM configuration on the source host is removed.

References

• Windows Server 2008 R2 & Microsoft Hyper-V Server 2008 R2 - Hyper-V Live Migration Overview & Architecture: http://www.microsoft.com/downloads/details.aspx?FamilyID=FDD083C6-3FC7-470B-8569-7E6A19FB0FDF&displaylang=en

Useful Links

• Windows Server 2008 R2 Virtual Machine processor compatibility mode: http://download.microsoft.com/download/F/2/1/F2146213-4AC0-4C50-B69A-12428FF0B077/VM%20processor%20compatibility%20mode.doc
• Hyper-V: Using Live Migration with Cluster Shared Volumes in Windows Server 2008 R2: http://go.microsoft.com/fwlink/?LinkId=139667
• Microsoft Support Policy for Windows Server 2008 Failover Clusters: http://support.microsoft.com/default.aspx?scid=kb;EN-US;943984