So, you got yourself a shiny new Windows Server 2012 R2 Essentials VM. You run ATTO Disk Benchmark 2.47, only to discover some of the VM's drives are on datastores that aren't quite up to speed. Worry not! Just use Storage vMotion, and the Advanced option, to take care of it. No downtime required, for most situations.
This video is in contrast to my recently published Resolve old VM’s 2TB drive size limitation with Windows Server 2012 R2 Essentials “Move the folder” video. This time around, instead of moving data from one drive to another at the OS level inside the VM, we'll do it from outside the VM using the vCenter. I'm using vSphere Web Client to tell ESXi 5.5 that we want to move all of a VM's drive's data from one VMFS datastore's sluggish drive to a faster one.
You'll see in the video below that I had a USB 3.0 PCI card passed through to this virtual machine, which prevented Storage vMotion. This is one of the drawbacks to VT-d. But you'll see it's easily resolved by removing the PCI device from the VM, followed by a quick reboot of the VM. I was then able to do the selective Storage vMotion. It may take hours if it's terabytes of data to move, but at least your VM can be busy doing other things, simultaneously.
The video below is a bit rough, with an actual look at exactly this kind of drive data rearranging that I performed on my actual home lab's vZilla. Not an artificial demonstration, but a real look at me doing home lab adjustments. Enjoy!