I just wanted to add that according many testimonies, like: https://lists.isc.org/mailman/htdig/bind-users/2011-February/082814.html, BIND9 with LDAP over DLZ has a very low performance, making it unsuitable for production systems, which is not the case with PowerDNS.
I too, read that before we rolled out our DNS cluster, but when we came to trying it ourselves, we got completely different results, or perhaps, acceptable results. Sure LDAP+DLZ was not quite as fast as BDB+DLZ, but the latter had so many troubles it was not worth it. We migrated from BDB+DLZ to LDAP+DLZ.
We now have one ldap-master, and 6 ldap+bind9+DLZ servers. (3+3 in two data centers). We have had very few troubles with this setup. (troubles are related to syncrepl). Authoritative only, no recursion. No servers in front. Since the BIND+DLZ servers use slapd on localhost, they continue to function if isolated (network troubles etc).
All DNS records are instantly updated, including DynDNS service. No restarts ever required. We are very happy with this setup. Currently, it is hosting 150183 domains (not counting sub domains, or records).
A random host lookup responds in:
25ms, 1ms, 1ms.
Which is acceptable to us.
Lund