Key resources to prevent VM live migration failures

Large or heavily used VMs present administrators with special challenges when it comes to live migration.

Are there any special requirements or processes to keep in mind for migration of large or heavily used virtual machines?

When it comes to optimizing the VM live migration process, most of your efforts should focus on memory and network connectivity.

First, remember the overall host workload. It is important not to fully utilize the source or destination host if you plan to perform a VM live migration. Obviously, the destination host must have sufficient free resources to accommodate the inbound VM, but there is more to it than that. The migration process requires the hypervisor to use memory and CPU resources. An optimal migration depends on the availability of sufficient host stores.

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Another consideration is the hardware resources that have been allocated to the VM. Migrating a running VM involves copying pages of memory. If a VM is being heavily utilized, then it is possible that memory pages may be modified faster than they can be copied, which could result in a failed migration.

Stun During Page Send (SDPS) is a feature in vSphere5 that can slow down a VM during the migration process and help keep memory transaction backlog from becoming problematic. However, it is usually better to make sure in advance that your VM is equipped to keep pace with demand, rather than resort to slowing down the VM.

Both VMware and Microsoft offer best practices for VM live migration that are tailored to their own platforms. VMware's vMotion Best Practices are available online. Microsoft also maintains a best practices checklist online for Hyper-V.

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