Tonight I found myself recovering a VM on a Windows 2008 R2 cluster connected to an MD3000i SAN. To make very long and painful story short, it appears that someone applied a series of Windows Updates and then a Dell DSM update that screwed up the multipathing.
In the end, I resorted to breaking the cluster and then mounting the LUNs. Most of the VMs I kept on one server, but I wanted to move a handful to what used to be the other node of the cluster. However, when reassigning the LUN in the MD Client to another host, I received an error that there was already a persistent reservation on the volume and that I should talk to my administrator.
In order to free the SCSI-3 reservation on the LUN, I had to run this command:
smcli -n -c “clear virtualDisk [\”virtualDiskName\”] reservations
After that, I was able to successfully re-assign the LUN.