Unsynchronized VBO Reads?

[Novakog]

Call `glReadPixels`. Or `glGetTexImage`. Or whatever command you're doing that copies pixels from an image into the buffer object.

Sorry - I think you may have misread my post? I'm talking about using VBOs for drawing geometry, so there isn't a read command.

[Novakog]

In the scenario described above the buffer is not modified at all by the GPU. In that case would you sync on the last glBuffer(Sub)Data or rather on the last draw command using (=reading from) that buffer ?

That's correct, the GPU should not be modifying the buffer at all in this case, so I would expect the sync to be on the last glBuffer(Sub)Data. But based off the experimentation and performance differences, I think that ATI/AMD on Windows may be syncing with drawing (essentially treating the buffer as having a basic lock, rather than a read-write lock).