[mythtv] How does mythtv decide which backend does the recording?

[cmisip]

I have two backends, one of them is out of space.  I had assumed that
mythtv would select the second backend for recording, however I found a
number of nuv files with zero size in the backend that was out of hard
disk space.  Does mythtv take into consideration the amount of hd space
when deciding which backend should do the recording.  If not this would
be a useful addition.  Thanks.