The issue lies in the fact that most of the common SAN devices (including EQL, I'm not familiar with netapp,etc) do not allow for the zeroing of nor downsizing LUNs- not to mention resizing a LUN is considered dangerous on most platforms (I've never attempted it) and regardless requires detaching the LUN in question.

Say you thin provision 10 100g disks onto a 1TB LUN expecting them to never go anywhere near their "available" 100g. Something funky happens, maybe a user dumps their iTunes database into their share, whatever. Months will go by and this will snowball unless the environment is very strictly monitored.

I've had LUNs brought down before when I first began working in these environments, in fact that error is from one of my Equallogics years back. Thank god I had a TON of snapshot reserved space that I was able to wipe out to bring every LUN back online.

FWIW, I honestly have no idea how to recover from a LUN going offline if there is no reserved space and you can't zero out the LUN unused-space. If you can't zero it out, then even if your vmotion some VMs off of it onto another SAN you're still screwed because the SAN doesn't realize that it has any free space.

Anyone have any insight? All I can think of is shutting down every single VM, unmounting the LUN, resizing it via the SAN CLI and hoping none of your vmfs data blocks are touched. Is there a simple DRS option available? VAAI?

What I've learned to follow is "You can always size up.. you can't size down"