Moving a vmdk from one dir to another on the same datastore should be almost instantly and does not require extra space.
How do you move vmdks ?
I attempted to use the datastore browser. My intent was to move a VMDK from one folder to another on the same datastore. During the process, the available space on the datastore began to decrease. Eventually the process was stopped by VMware. My guess is it stopped around the time the datastore was showing 95% full.
I would have expected the "Move to" option to not eat up free space. The move operation must copy the file and then delete the source when it successfully completes.