Storage vMotion involves moving a virtual machine's files from one datastore to another. Using this technology, you can move a virtual machine between storage devices to allow for maintenance or reconfiguration of the original storage device without any kind of virtual machine downtime.

Outwardly, everything in Storage vMotion looks identical to what it used to look like in the case of the administrator; however, it operates differently under the hood in vSphere 5.x. Now, when a Storage vMotion migration is initiated, a new process, known as mirror driver, runs on the ESXi host. The mirror driver ensures that, as blocks change at the source datastore during Storage vMotion migration, it writes the changes to both datastores simultaneously.

Depending on the type of array backing the datastore, either the Data Mover built into the VMkernel or the vSphere Storage APIs—Array Integration (VAAI)— will be used to copy the blocks to the destination. VAAI, sometimes...