vSphere vMotion, vSphere Storage vMotion, vSphere DRS, vSphere Storage DRS, Storage I/O Control, vSphere 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 ESXi. With vMotion, you can migrate running virtual machines from one physical server to another without service interruption. 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.

With Storage vMotion, you can migrate virtual machines from one datastore to another datastore without service interruption. This ability allows administrators, for example, to off-load virtual machines from one storage array to another to perform maintenance, reconfigure LUNs, resolve 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.

vSphere Distributed Resource Scheduler (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 so that the 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 uses vMotion to migrate virtual machines to other hosts as necessary.

vSphere 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 you add a new physical server to a cluster, DRS enables virtual machines to immediately take advantage of the new resources because it distributes the running virtual machines.

When distributed power management (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. When 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 perform virtual machine placement, virtual machine migration, and host power actions, or to provide recommendations that the datacenter administrator can assess and manually act on.

Storage DRS helps you manage multiple datastores as a single compute resource, called a datastore cluster. A datastore cluster is an aggregation of multiple datastores into a single logical, load-balanced pool. You can treat the datastore cluster as a single flexible storage resource for resource management purposes. In effect, a datastore cluster is the storage equivalent of an ESXi compute cluster. You can dynamically populate datastore clusters with datastores of similar characteristics. You can assign a virtual disk to a datastore cluster and Storage DRS finds an appropriate datastore for it. The load balancer manages initial placement and future migrations based on workload measurements. Storage space balancing and I/O balancing minimize the risk of running out of space and the risk of I/O bottlenecks slowing the performance of virtual machines.

Storage DRS
Storage DRS can migrate virtual disks based on workload

Storage I/O Control congestion management allows cluster-wide storage I/O prioritization. You can control the amount of storage I/O that is allocated to virtual machines during periods of I/O congestion, which ensures that more important virtual machines are preferred over less important virtual machines for I/O resource allocation.

With vSphere HA, virtual machines automatically restart on a different physical server in a cluster if a host fails.

vSphere HA monitors all physical hosts in a cluster and detects host failures. Each physical host maintains a heartbeat with the other hosts in the cluster. Loss of a heartbeat initiates the process of restarting all affected virtual machines on other hosts. vSphere HA admission control ensures that if a host fails, sufficient resources are available in the cluster at all times to restart virtual machines on different physical hosts.

vSphere HA
VMware High Availability

vSphere HA also provides a Virtual Machine Monitoring feature that monitors the status of virtual machines in a vSphere HA cluster. If a virtual machine does not generate heartbeats within a specified time, Virtual Machine Monitoring identifies it as having failed and restarts it. If restarts occur, policies can control the number of restarts. Similarly, you can use the Application Monitoring feature. If the heartbeats for an application are not received for a specified time, Application Monitoring restarts its virtual machine.

vSphere Fault Tolerance on the ESXi host platform provides continuous availability by protecting the primary virtual machine with a secondary virtual machine that runs simultaneously on a separate host. Inputs and events performed on the primary virtual machine are recorded and replayed on the secondary virtual machine, which ensures that the two remain in an identical state. For example, mouse-clicks and keystrokes are recorded on the primary virtual machine and replayed on the secondary virtual machine. Because the secondary virtual machine is running simultaneously with the primary virtual machine, it can take over at any point without service interruption or loss of data.

For more information on vMotion and Storage vMotion, see the vCenter Server and Host Management documentation. For more information on DRS, HA, and Fault Tolerance, see the vSphere Availability documentation.