VMware VMotion, VMware Storage VMotion, VMware DRS, VMware HA, and Fault Tolerance are distributed services that enable efficient and automated resource management and high availability for virtual machines.

Virtual machines run on and consume resources from ESX/ESXi. VMotion enables the migration of running virtual machines from one physical server to another without service interruption, as shown in Migration with VMotion. The effect is a more efficient assignment of resources. With VMotion, resources can be dynamically reallocated to virtual machines across physical servers.

Migration with VMotion
This image shows how VMotion migrates a virtual machine from one server to another.

Storage VMotion enables the migration of virtual machines from one datastore to another datastore without service interruption. This allows administrators, for example, to off-load virtual machines from one storage array to another to perform maintenance, reconfigure LUNs, resolve out-of-space issues, and upgrade VMFS volumes. Administrators can also use Storage VMotion to optimize the storage environment for improved performance by seamlessly migrating virtual machine disks.

VMware DRS helps you manage a cluster of physical hosts as a single compute resource. You can assign a virtual machine to a cluster and DRS finds an appropriate host on which to run the virtual machine. DRS places virtual machines in such a way as to ensure that load across the cluster is balanced, and cluster-wide resource allocation policies (for example, reservations, priorities, and limits) are enforced. When a virtual machine is powered on, DRS performs an initial placement of the virtual machine on a host. As cluster conditions change (for example, load and available resources), DRS migrates (using VMotion) virtual machines to other hosts as necessary.

VMware DRS
This graphic demonstrates how VMware DRS moves virtual machines within a cluster from one server to another. The graphic displays three physical servers that share a cluster. The cluster contains three ESX servers. each containing one or more virtual machines. One virtual machine is shown moving from one server to another.

When a new physical server is added to a cluster, DRS enables virtual machines to immediately and automatically take advantage of the new resources because it distributes the running virtual machines.

When DPM is enabled, the system compares cluster-level and host-level capacity to the demands of virtual machines running in the cluster. If the resource demands of the running virtual machines can be met by a subset of hosts in the cluster, DPM migrates the virtual machines to this subset and powers down the hosts that are not needed. Once resource demands increase, DPM powers these hosts back on and migrates the virtual machines to them. This dynamic cluster right-sizing that DPM performs reduces the power consumption of the cluster without sacrificing virtual machine performance or availability.

You can configure DRS to automatically execute virtual machine placement, virtual machine migration, and host power actions, or to provide recommendations which the datacenter administrator can assess and manually act upon.

VMware HA enables quick restart of virtual machines on a different physical server within a cluster automatically if a host fails. All applications within the virtual machines have the high availability benefit, through application clustering.

HA monitors all physical hosts in a cluster and detects host failures. An agent placed on each physical host maintains a heartbeat with the other hosts in the resource pool. Loss of a heartbeat initiates the process of restarting all affected virtual machines on other hosts. See VMware HA for an example of VMware HA. HA ensures that sufficient resources are available in the cluster at all times to restart virtual machines on different physical hosts in the event of host failure.

VMware HA
VMware High Availability

HA also provides a Virtual Machine Monitoring feature that monitors the status of virtual machines in an HA cluster. If a virtual machine does not generate heartbeats within a specified time, Virtual Machine Monitoring identifies it as having failed and automatically restarts it. If restarts occur, policies can control the number of restarts.

With HA, vCenter Server is not a single point of failure. HA is configured centrally via vCenter Server; however, once configured, it operates continuously and in a distributed manner on every ESX host. vCenter Server is no longer involved. Even if vCenter Server fails, HA failovers still occur successfully.

Using VMware vLockstep technology, VMware Fault Tolerance (FT) on the ESX/ESXi host platform provides continuous availability by protecting a virtual machine (the Primary VM) with a shadow copy (Secondary VM) that runs in virtual lockstep on a separate host. Inputs and events performed on the Primary VM are recorded and replayed on the Secondary VM, ensuring that the two remain in an identical state. For example, mouse-clicks and keystrokes are recorded on the Primary VM and replayed on the Secondary VM. Because the VM is in virtual lockstep with the Primary VM, it can take over execution at any point without interruption or loss of data.