From the document:

Overview of Live Storage Migration

Virtual disks can be migrated from one storage domain to another while the virtual machine to which they are attached is running. This is referred to as live storage migration. When a disk attached to a running virtual machine is migrated, a snapshot of that disk’s image chain is created in the source storage domain, and the entire image chain is replicated in the destination storage domain. As such, ensure that you have sufficient storage space in both the source storage domain and the destination storage domain to host both the disk image chain and the snapshot. A new snapshot is created on each live storage migration attempt, even when the migration fails.

Consider the following when using live storage migration:

You can live migrate multiple disks at one time.

Multiple disks for the same virtual machine can reside across more than one storage domain, but the image chain for each disk must reside on a single storage domain.

You can live migrate disks between any two storage domains in the same data center.

You cannot live migrate direct LUN hard disk images or disks marked as shareable.

But where do users perform online storage migrations?

There seems to be no interface.