Once upon a time, whilst working for a fairly well-known UK domain registration company, I put together a system built on an early version of the BIND-DLZ patchset against BIND 9.2.5 (If I recall correctly). It used MySQL as the backend database (because that's what the registration system used for CRM purposes) and worked very nicely, thankyou, for well in excess of a million zones and a query rate which I forget but was of the order of several thousand per second, maybe higher at times. We had a custom-written web management toolbox, part of which was exposed to customers through their control panel so they could manage their zones by themselves. The "frontend" nameservers - those actually answering queries - had a "read only" one-way replicated copy of the tables being managed by the CRM system, so all changes were near instantaneous. Copious caching options and indexing in MySQL gave the DB pretty good performance. The frontend servers themselves were load balanced and fault-tolerant and in theory at least a single machine could handle the overall system load. Unfortunately, after I moved on from that job the system broke in some spectacular way (I don't know why) and has since been significantly changed from the original spec, but I couldn't say how... DLZ worked for us - but the DB and management tools were built "in house"; I don't think there's an ideal off-the-shelf solution built around it (yet). Graeme