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.