VMware launched live migration in 2003. Since then, its capabilities have seen many enhancements, but Microsoft is starting to catch up.
Live migration entails moving active virtual machines (VMs) between physical hosts with no service interruption or downtime.
It launched 11 years ago as a landmark development in datacentre infrastructure and is now a crucial part of virtualisation infrastructure software and deployment.
A VM live migration allows administrators to perform maintenance and resolve a problem on a host without affecting users.
Moving active VMs from one hypervisor to another means you can balance the performance and load of hypervisors or, in the case of hardware maintenance, evacuate hypervisors from active VMs. It enables users to conserve resources during non-peak hours by moving VMs to fewer servers. You can also optimise network throughput by running VMs on the same hypervisor. When live migration of VMs appeared in 2003, with VMware’s ESX 2.0, it became popular in the IT community.
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Six years after VMware pioneered VM live migration in 2003, Microsoft introduced a similar feature in Hyper-V that was shipped with Windows Server 2008 R2 – the previous version, Quick Migration, in Windows Server 2008 required a short service interruption during migration.
Getting to grips with live migration
To understand how live migration works, it is important to be aware of the VM’s basic components: storage (the virtual hard disk) and the configuration or state. Storage is often located on a storage area network (SAN) and its configuration runs in a host server’s processor and memory. With the traditional process of a live migration, the VM’s state and configuration is copied from one physical host to another, but the VM’s storage does not move.
Shared nothing live migration is a combination of traditional VM live migration and storage migration
Storage live migration – moving the disks of a VM from one location to another while the VM continues to run on the same physical host – became available in 2006/2007 with ESX 3.0/3.5. VMware’s current offering, vSphere 5.5, vMotion (live migration of VMs) and Storage vMotion (live migration of the virtual disks) are part of the vSphere standard edition. Automatic load-balancing of VMs (distributed resource scheduling, or DRS) is available with vSphere Enterprise, automatic load balancing of disks (Storage DRS) and vSphere Enterprise Plus. Leveraging vMotion requires that ESXi servers are being managed by Virtual Center and that they are compatible (boiling down to compatible CPUs and a couple of minor requirements) with the same physical subnet.
Moving VMs between hypervisors that are not on the same physical network segment is not supported. Administrators need to tag an existing port or create a new VMkernel port for vMotion usage and live migration to be used by with one click in the vSphere Client (either the traditional client or the web client). Using the web client, even live migration of VMs without shared storage is possible (shared-nothing live migration, introduced with vSphere 5.1).
Shared-nothing live migration is a combination of traditional VM live migration and storage migration. The VM’s state and configuration is copied to a destination host and the file system is moved to the destination storage device. To prevent downtime, the VM’s state and storage remain running on the original host and storage location until the copying process is completed.
Evolution of VMware’s live migration
VMware has improved its live migration capabilities over the years and the application can now leverage multiple network interfaces to speed up live migration. In VMware’s upcoming vSphere 6, rumoured to be launching in March 2015, live migration over longer distances – with higher latencies and between virtual centre instances – is expected to be available.
DRS, which leverages vMotion to balance VM workload between physical hosts, has also been improved in recent product versions. It now boasts rules that take preferences into account and can evacuate hypervisors during non-peak hours to conserve resources using distributed power management (DPM), available with DRS as part of vSphere Enterprise Edition. VMware also updated Storage vMotion in vSphere version 5.0 by moving from a dirty block tracking algorithm to I/O mirroring, improving the performance and reliability of its storage live migration capabilities.
Microsoft stealing VMware’s thunder
Microsoft introduced the ability to move VMs across Hyper-V hosts with Windows Server 2008 R2. This required VMs to reside on shared storage as part of a cluster. Even then, Hyper-V wasn’t able to move multiple machines simultaneously. However, with Windows Server 2012 and Server 2012 R2, Microsoft continued to gain ground on VMware, introducing additional migration capabilities that put Microsoft more or less on par with VMware when looking at this specific feature.
Since Windows Server 2012 R2 Hyper-V can store VMs on server message block (SMB) file shares, performing live migration on running VMs stored on a central SMB share is now possible between non-clustered and clustered servers, so users can benefit from live migration capabilities without investing in clustering infrastructure. Windows Server 2012 R2’s live migration can also leverage compression, reducing the time needed to perform live migration by 50%, according to Microsoft.
Live migration in Windows Server 2012 R2 can use improvements in the SMB 3.0 protocol too, which accelerate live migration without the VM having to be stored on a SMB 3.0 share. If the customer is using network interfaces that support remote direct memory access (RDMA), the flow of live migration traffic is faster and has less impact on the CPUs of the hosts involved.
Storage live migration was introduced to the Hyper-V feature set with Windows Server 2012. Windows Server 2008 R2 allowed users to move a running VM using traditional live migration, but you had to shut down a VM to move its storage in Windows Server 2008 R2. With the current version of Hyper-V, you can transfer a VM’s backing storage files to a new location with no downtime, a feature that is critical for migrating or updating storage, or when a load redistribution on the storage side is needed.
VMware vSphere 5.5 versus Microsoft Windows Server 2012 R2 Hyper-V
In their current versions, VMware’s vSphere 5.5 and Microsoft Windows Server 2012 R2 Hyper-V support shared-nothing live migration, which makes it possible to simultaneously change the location where the VM is being run as well as the backing storage location for the running VM – a feature that provides additional flexibility, especially in small business environments where centralised storage is not always present.
Microsoft has gained substantial ground in many areas, but experts agree there is still a gap between Hyper-V and VMware vSphere when looking at enterprise-level features. Hyper-V lacks features, such as vSphere Storage DRS, though other features, such as Storage Spaces, offer similar functionalities. But Hyper-V comes in a powerful free version, “Hyper-V Server 2012”, which includes native support for Live Migration of VMs across clustered and non clustered hosts at no extra cost, while VMware’s free hypervisor has limited functionality. Going beyond live migration, both suppliers support replication capabilities, which is easy to set up with VMware vSphere and Microsoft’s Hyper-V. Combined with the cloud offerings of the suppliers, VMware vCloud Air Disaster Recovery and Microsoft Azure Site Recovery, users can replicate and failover VMs to their suppliers’ cloud offerings, giving extra options for self-service disaster recovery protection and business continuity.