The downtime involved with the failover depends on how much load the Data Mover is handling means how many FS are loaded and other things that are mounted on the DM. It may vary from 2 minutes to 15 minutes. As far as the VMs are concerned they should come up without any issues (with some inaccessibility while server_3 take over). If there is any issue with the VMs loading. Please contact support team. 
I am currently at a point where i need to failover Server_2 to Server_3 so the Server_2 can be rebooted. I have many VMWare VMs (vSphere 4) running via software iSCSI on this NX4 device and i am curious if anyone has done this and what happens to their VMs running when you do the failover and failback. I have tested it with a second NX4 box here but only with a couple VMs running at 1 time. They seem to go out for about 10 Seconds and them come right back just as they were. Is this supposed to be true also if i have about 30 VMs running from the production NX4 device? Will they come back or will they bluescreen?   Any info would be great...thanks.
Datamover Failover and how it affects VMs