Multipath I/O feature
Multipath I/O (MPIO) is a feature of Windows Server 2008 that allows server to use multiple data paths to a storage device. This increases the availability of storage resource because it provides alternate paths from a server or cluster to a storage subsystem in the event of path failure. MPIO uses redundant physical path components (adapters, switches, cabling) to create separate paths between the server or cluster and the storage device. If one of the devices in these separate paths fails an alternate path to the SAN devices will be used, ensuring that the server is still be able to access critical data. You configure failover times through the Microsoft iSCSI Software Initiator driver or by modifying the Fiber Channel HBA driver parameter settings, depending on the SAN technology deployed in your environment.
\nNew MPIO features in Windows Server 2008 include a Device Specific Module (DSM) designed to work with storage arrays that support the asymmetric logical unit access (ALUA) controller model (as defined in SPC-3), as well as storage arrays that follow the Active/Active controller model.\n\nThis feature can be installed via Add Features wizard or via ServerManagerCmd.exe: ServerManagerCmd.exe -install Multipath-IO
Multipath I/O Overview: http://technet.microsoft.com/en-us/library/cc725907.aspx