Whatever topic has been discussed on this blog is my own finding and views, not necessary match with others. I strongly recommend you to do a test before you implement the piece of advice given at my blog.
We noticed around 1.6TB storage difference between two Eight (1/8) Exadata boxes while configuring Data Guard. Wondered what went wrong. The Exa box configured for DR was around 1.6TB short compare to the other Exa box. Verified the lun, physical disk and grdidisk status on a cell, which showed active/online status. The tricky part on Exadata is, everything has to be active/online across all cell storage servers. We then figured-out that grid disk status on the 3rd cell storage server was inactive. After making them active on the 3rd cell server, everything become normal, i mean, the missing 1.6TB space appeared.
When you work with Exadata, you need to verify all cell storage servers to confirm the issue, rather than just query things over just one cell server.