A while back I presented a diagram showing how storage is lost throughout the provisioning process.
I've added a few more items onto the diagram and heres version 2. The additions show reasons why storage is lost at various points in the cycle, for example, disks not not in use, hot spares, not using all the remaining space on the disk etc.
If anyone has additional reasons I've missed, then please let me know.
The next step is to look at ways of retrieving this storage and improving efficiency.