When we first setup ESXi, we over allocated drive space (allocated 300 GBs of a 250GB hard drive) thinking ESXi would shuffle things intelligently like it does with memory and CPU but found out later it doesn't work that way. We were told there was no way to reduce the allocations without deleting the associated drive and major problems could occur if we use more of the drives than exist.
But today, I found I was able to easily extend a drive without even rebooting and was hoping a could use the same technique to shrink a drive. I was able to shrink the drive through Windows 2008 but when I try to decrease the provisioned size through vSphere, it doesn't let me. In the disk partitioning section, it does show the type as "thin".
Would I be able to decrease the allocation if the VM were powered off? If not, if I just shrink the drives in Windows 2008 below the actual available space, will that resolve the possible problem with more space being allocated than actually exists?