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An Overview of Hyper-V Live Migration

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In the previous chapter we looked at the the migration of Hyper-V virtual machines that required the virtual machine to be powered off or in placed into a saved state. Microsoft, oddly enough, refer to this technique as ''Quick Migration''. The downside to this approach is that it is a manual process and any service provided by the virtual machine is interrupted during the migration process, hardly an acceptable solution in mission critical and enterprise level environments.
Fortunately Hyper-V 2008 R2 introduced a feature called ''Live Migration'' whereby a running virtual machine can be migrated from one host system to another with no interruption in service.
In this chapter we will look in detail at the system requirements and steps in involved in performing a live migration using Hyper-V 2008 R2.
== Hyper-V Live Migration Configuration Requirements ==
Before a running virtual machine can be migrated from one host to another there are some mandatory requirements thant must first be met:
- Hyper-V 2008 R2 must be deployed on both hosts. The first version of Hyper-V does not support live migration.
- Source and destination Hyper-V hosts must be configured as a Failover cluster.
- Source and destination systems must be using shared storage (i.e. via SAN or iSCSI configurations)
- Source and destination systems must be running processors from the same manufacturer. It is not, for example, possible to migrate a virtual machine from an Intel based host to one containing an AMD CPU.
== The Live Migration Process ==
The live migration process performs the following tasks:
- '''Configuration transfer''' - The source host transfers the configuration data for the virtual machine to the destination host.
- '''Destination VM creation''' - Based on the configuration data the destination host creates a new virtual machine and allocated adequate memory.
- '''Transfer of memory pages''' - The memeory pages used by the virtual machine on the source host are transferred to the destination host. During this process, the source host monitors the memory and records any changes that take place after each page is transferred. Modified pages are then resent to the destination host.
- '''Transfer of state''' - Register and device state are sent to the destination server.
- '''Transfer of storage''' - The source host transfers the shared storage handle to the destination host where it is used to access the virtual hard disks (VHDs) and passthrough disks required by the virtual machine.
- '''VM online''' - The migrated virtual machine is brought online on the destination host.
- '''Network Re-direction''' - The network environment is modified to direct traffic intended for the virtual machine to the destination host.

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