>    It seems there should be a way for the parity checking to be
   >    done in the background.
   > 
   > It can.  Just run raidctl in the background.

   But doesn't the raid device become unavailable?  (since the
   device is busy)?

I'm pretty sure it "just works."  I suspect that until parity is
regenerated you will be running in a degraded mode with no access to
the parity information anyway.  Regenerating parity from the other
parts doesn't interfere with that.  Once parity is fine, it switches
to full raid operation.

Cheers,
Brook