What is the capacity of a BD-R disk really?

[Tom Horsley]

I thought I would test my blu-ray writer by backing up a bunch
of big files to a BD-R. I used k3b which imagines there is 25GB
of available space, so I added files till I was about to
overflow that.

Then I insert a blank BD-R disk (and hit bugzilla 742719), but
I go ahead and tell it to burn, and growisofs (I think) gives
an error saying that many files won't fit.

So, I've got k3b telling me 25GB before it sees the media,
2.1TB after it sees the media, and growisofs not telling
me a specific size, but saying I'm trying to write too much :-).
It would sure be nice if there were a consistent answer here.

Anyway, I keep pulling the smallest files out of the list
till growisofs finally thinks it can burn the BD-R, and that
runs for a while till it gets to about 98%, then gives
an error saying the files won't fit, so the check it was
doing earlier wasn't quite right. The last file it wrote
gives I/O errors if I try to read it (but at least I can
verify the checksums of all the other files).

So, does anyone have any pointers to information on the
actual capacity of BD-R disks? None of the tools on fedora
seem to agree, and none of them appear to be correct
either :-). (And web searches are remarkably fruitless
on this topic).

Is BD-R back to the days where you have to do a simulated
write before you can get an actual answer? Or is it simpler
just to provide about 3GB of slop to be sure everything fits?