Here is the KB that was just published those days : support.microsoft.com/.../en-us
Hi all,  Just a piece of information for those who are using Windows 2008 R2 MPIO feature with Round Robin with Subset policy.  Our context is : 2 Compellent Array in replication, used in Live Volume mode. Each server see 4 SAN paths to the primary array and 4 SAN paths to the secondary Array. According to the Live Volume Best Practises, we configure our MPIO policy to use the Round Robin with subset in order to have the first 4 paths ACTIVE and the last 4 paths STANDBY. If you use these "RRWS" policy with Windows 2008 R2, you should now that there is a bug, confirmed to me by Microsoft Premier Support : the ACTIVE/STANDBY mode configured on each paths is not persistent across server Reboot (after reboot, only the first path remains active). These bug was known by Microsoft but was only corrected on Windows 2012. The patch for Windows 2008 R2 is about to be delivered. Regards,
INFO : Windows 2008 R2 - MPIO feature bug confirmation for Round Robin With 
Subset Policy