I have tried to load 1 million entries into OpenLDAP on Redhat6.2(P-III, 800MHz, 512M RAM,
ldbm with berkeley DB, about 10 attributes indexed), it taken about 6 days long.
With Netscape Directory Server, it taken about one and half days. Netscape takes some special
actions when the size of .db files are too large, however, the OpenLDAP does not.
For 20 Million entries, the only way is to distribute your database into serveral servers,
ex. 500K entries per server. If you can adopt distribution well, the OpenLDAP is also suitable
for Large database.