[Zope] newbie: why external database?

[frank sagel]

ahoi,

I want to develope a forum with zope. There are products wich store
messages and replies in the ZODB. In case this forum will become
crowded, this seems to me not the right way ( ? ). I've read that ZODB
does a good job if 99% of the users only read. Is this right? Can
somebody tell me at wich "size" (e.g. amount of users, hits, etc... ) an
external db is needed?

Anyway if i'd have to migrate to an external db, how is this
functioning?

thanks for replies

frank