OpenKM DBMS works as you said, metadata etc... are on one DBMS and binary information in other, normal configuration directly at hard disk structure ( can be stored on DMBS but is not the configuration that comes by default ). We use rsync for incremental backup ( consider you need metadata too, because on hard disk binaries are stored in non human file name and folders structure, you will not recognise by name etc... as files you see in OpenKM, uuid etc... is used to store binary files ok ?
About using community version for greater repositories like 1 Tera etc.. I do not suggest this scenario, althought I know there are users who are using greater repositories with community version ( 200Gb and something more, I have no notice for 1 tera or upper ), professional version is optimized for 1 milion documents and more, but not community. About question, can I use community for it ?, the answer is yes, but you should be working on optimization if you get performance problems, study database queries time, etc... Community version is prepared for general purpose users, effors are orientated in making extensible to almost users, to solve general documentation problems, focused on little and small companies with general purpose repositories ( store company documentation etc... ) we considering 100k documents the maximum scenario for the 90% of candidate users.
Basically for understanting us, we dedicate time to solve general problems to arrive at major number of users. We must decide where we dedicate effors, new general purpose feature or exclusive one for one company or with less candicates. In this case we select general purpose. The idea is that what is not general purpose, as you can have source code, you can collaborate in making openkm better contributing with your specifics needs, optimizations. Some people does it, other dedice contract us for doing it, this case is professional version, where we make optimizations or special parametrizations for our customers. Basically this is the idea. Sincerally consider 1 milion docs is not into the general problem, really upper 100K you need working on optimizations.