Quick Storage Migration (QSM) enables the migration of the storage of VM from one location to another. For example, suppose you have virtual machines on a leased SAN (SAN 1). The lease runs out and you decide to upgrade to a new SAN (SAN 2) with more capacity, better performance and additional capabilities. Quick Storage Migration allows you to move the virtual machine which resides on SAN 1 to SAN 2.

QSM relies on Windows Server 2008 R2 Hyper-V and Background Intelligent Transfer Service (BITS). QSM can move the virtual disks of a running virtual machine independent of storage protocols (iSCSI, FC) or storage type (local, DAS, SAN), with minimal downtime. Under 1 minute of downtime in most cases.

Read the post on QSM, see some comparisons, and get a detailed understanding of how it works on the Microsoft Virtualization Team blog: http://blogs.technet.com/virtualization/archive/2009/06/25/system-center-virtual-machine-manager-2008-r2-quick-storage-migration.aspx