Red Hat Enterprise Virtualization live migration is a powerful management and maintenance tool for data centers of all sizes. Whether you're trying to trim electricity costs or need to execute a planned migration, RHEV live migration will come into play.
Live migration is the process of moving a virtual machine (VM) between hosts without a service disruption. RHEV enables live migration by default, and it requires only a minimum of two RHEV hosts connected to shared storage. So if you have a few hosts attached to an iSCSI SAN, you're ready to start using RHEV live migration. There's no need to configure anything else in your cluster.
Manual and automatic RHEV live migration
Live migration can be triggered manually or by specific events. To manually live migrate a VM, select it in the RHEV-M console and click the Migrate button. Next, you can specify which host to migrate the VM to or have RHEV make the decision automatically, in which case it will try to evenly distribute the workloads across the cluster. When the VM is in the process of being migrated, you can continue working on it and experience no interruption at all.
More on live migration
KVM supports live migration without shared storage
Hyper-V 3.0 to feature shared-nothing live migration
Overview of VMware vMotion
RHEV live migration is particularly useful when performing maintenance on a node. Just put the host in maintenance mode and all of its virtual machines will migrate to other hosts in the cluster. From there, you can work on the host without any service disruption to your VMs.
Once you bring a host out of maintenance mode, VMs will not automatically migrate back to the host because the default load-balancing policy for each host is set to none. Many administrators don't like VMs spontaneously moving around. If you want to change this policy, just set the load-balancing policy to Even Distribution, in which RHEV will try to run an even amount of VMs on all of the hosts.
Cutting electricity costs with RHEV
RHEV's Power Saving feature also uses live migration. With Power Saving enabled, RHEV will automatically migrate workloads in an attempt to run the hosts in the most power-efficient manner possible. The user sets thresholds for the minimum and maximum number of VMs that a server can host. If a host falls below the minimum threshold, the remaining VMs will move to another host because it's not efficient to keep the host powered on. If the number of VMs exceeds the maximum threshold, RHEV will move VMs off the host to ensure each workload has enough resources.
Ultimately, live migration is a powerful and necessary tool in every virtualization administrator's tool chest. Optimizing a virtual infrastructure without service disruptions allows IT pros to proactively manage a data center and keep end users happy.