[xwiki-devs] Documentation about the DB-migration?

Hello fellow XWiki developers, At Curriki, we are considering more and the possibility to migrate to the file-sytem attachment, however, the current solution (which does it at boot time) is not acceptable since it would make our service unavailable for several days (there are 73756 attachments on 591000 documents, when we tried on a dev server, it took a week). Where can I read more about the DB-migration that converts the DB attachments to the file-systems' attachments? Where is the source? What modules are used? thanks in advance. Paul