I put this into the beginners-section depite of reading some discussion about that issue.
If working with opaque buffer-objects the question arises whether the gl-implementation will Keep data set by BufferSubData at Hand. Not knowing this (assuming that gl just sends it away and Forgets about it) one is tried to keep a copy of the data oneself. Actually this may result in the data being around three-times: One copy in the app's Memory, one copy that the gl-implementation may Keep at Hand for itself and one copy residing on the graphics-board. The same goes for textures.
Does someone know (or have a good guess about) the policies gl-implementations have in that manner?