Jason Boche of boche.net comments:
One of the features that has been updated in vSphere 5.5 is UNMAP which is one of two sub-components of what I’ll call the fourth block storage based thin provisioning VAAI primitive (the other sub-component is thin provisioning stun). I’ve already written about UNMAP a few times in the past. It was first introduced in vSphere 5.0 two years ago. A few months later the feature was essentially recalled by VMware. After it was re-released by VMware in 5.0 Update 1, I wrote about it’s use here and followed up with a short piece about the .vmfsBalloon file here.
A nice long look at a new vSphere 5.5 storage feature that should help make things run a bit more smoothly.
Read more at: vSphere 5.5 UNMAP Deep Dive