In this article by Abhilash G B and Rebecca Fitzhugh author of the book Learning VMware vSphere, we are mostly going to be talking about howvSphere vMotion is a VMware technology used to migrate a running virtual machine from one host to another without altering its power-state. The beauty of the whole process is that it is transparent to the applications running inside the virtual machine. In this section we will understand the inner workings of vMotion and learn how to configure it.

There are different types of vMotion, such as:

Compute vMotion
Storage vMotion
Unified vMotion
Enhanced vMotion (X-vMotion)
Cross vSwitch vMotion
Cross vCenter vMotion
Long Distance vMotion

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Compute vMotion is the default vMotion method and is employed by other features such as DRS, FT and Maintenance Mode. When you initiate a vMotion, it initiates an iterative copy of all memory pages. After the first pass, all the dirtied memory pages are copied again by doing another pass and this is done iteratively until the amount of pages left over to be copied is small enough to be transferred and to switch over the state of the VM to the destination host. During the switch over, the virtual machine's device state are transferred and resumed at the destination host.You can initiate up to 8 simultaneous vMotion operations on a single host.

Storage vMotion is used to migrate the files backing a virtual machine (virtual disks, configuration files, logs) from one datastore to another while the virtual machine is still running. When you initiate a storage vMotion, it starts a sequential copy of source disk in 64 MB chunks. While a region is being copied, all the writes issued to that region are deferred until the region is copied. An already copied source region is monitored for further writes. If there is a write I/O, then it will be mirrored to the destination disk as well. This process of mirror writes to the destination virtual disk continues until the sequential copy of the entire source virtual disk is complete. Once the sequential copy is complete, all subsequent READS/WRITES are issued to the destination virtual disk. Keep in mind though that while the sequential copy is still in progress all the READs are issued to the source virtual disk. Storage vMotion is used be Storage DRS. You initiate up to 2 simultaneous SvMotion operations on a single host.

Unified vMotion is used to migrate both the running state of a virtual machine and files backing it from one host and datastore to another. Unified vMotion uses a combination of both Compute and Storage vMotion to achieve the migration. First, the configuration files and the virtual disks are migrated and only then the migration of live state of the virtual machine will begin. You can initiate up to 2 simultaneous Unified vMotion operations on a single host.

Enhanced vMotion (X-vMotion) is used to migrate virtual machine between hosts that do not share storage. Both the virtual machine's running state and the files backing it are transferred over the network to the destination. The migration procedure is same as the compute and storage vMotion. In fact, Enhanced vMotion uses Unified vMotion to achieve the migration. Since the memory and disk states are transferred over vMotion network, ESXi hosts maintain a transmit buffer at the source and a receive buffer at the destination. The transmit buffer collects and places data on to the network, while the receive buffer will collect data received via the network and flushes it to the storage. You can initiate up to 2 simultaneous X-vMotion operations on a single host.

Cross vSwitch vMotion allows you to choose a destination port group for the virtual machine. It is important to note that unless the destination port group supports the same L2 network, the virtual machine will not be able to communicate over the network. Cross vSwitch vMotion allows changing from Standard vSwitch to VDS, but not from VDS to Standard vSwitch. vSwitch to vSwitch and VDS to VDS is supported.

Cross vCenter vMotion allows migrating virtual machines beyond the vCenter's boundary. This is a new enhancement with vSphere 6.0. However, for this to be possible both the vCenter's should be in the same SSO Domain and should be in Enhanced Linked Mode. Infrastructure requirement for Cross vCenter vMotion has been detailed in the VMware Knowledge Base article 2106952 at the following link:http://kb.vmware.com/kb/2106952.

Long Distance vMotion allows migrating virtual machines over distances with a latency not exceeding 150 milliseconds. Prior to vSphere 6.0, the maximum supported network latency for vMotion was 10 milliseconds.

Using the provisioning interface

You can configure a Provisioning Interface to send all non-active data of the virtual machine being migrated. Prior to vSphere 6.0, vMotion used the vmkernel interface which has the default gateway configured on it (which in most cases is the management interface vmk0) to transfer non-performance impacting vMotion data. Non-performance impacting vMotion data includes the Virtual Machine's home directory, older delta in the snapshot chain, base disks etc. Only the live data will hit the vMotion interface. The Provisioning Interface is nothing but a vmkernel interface with Provisioning Traffic enabled on this. The procedure to do this is very similar to how you would configure a vmkernel interface for Management or vMotion traffic. You will have to edit the settings of the intended vmk interface and set Provisioning traffic as the enabled service:

It is important to keep in mind that the provisioning interface is not just meant for VMotion data, but if enabled it will be used for cold migrations, cloning operations and virtual machine snapshots. The provisioning interface can be configured to use a different gateway other than vmkernel's default gateway.

Further resources on this subject: