bind9 answers no queries during AXFR

[Martin H. Sluka]

We got a BIND9 server which is a secondary for a very large DNS
zone (an RBL; about three million entries, 70 MB as file, several
hundred MB in RAM) that usually updates each few hours.  Even on
current hardware, the zone transfer (or reading the zone after
the transfer) takes several minutes.

The problem is, that BIND9 does not seem to answer _any_ queries
during the update phase.  Any help to solve this problem would be
appreciated.  (Splitting the zone is, however, not possible.)

Regards,
Martin