That depends on exactly what kind of DBMS and underlying OS one is talking about. For DBMS that runs as one daemon or set of related daemons that has some kind

That depends on exactly what kind of DBMS and underlying OS one is talking about. For DBMS that runs as one daemon or set of related daemons that has some kind of mechanism to share the afore mentioned buffer pool and synchronize acces to it then sure, it makes more sense, especially as you can do fine-grained copy on write in that bufferpool as an transaction isolation mechanism. For embedded databases that operate on shared file somewhere just mmap()ing that file allows you to use part of the same file as shared memory segment and for POSIX IPC and thus synchronization between otherwise unrelated processes that access the same data. One big caveat of this approach is that it depends on optional parts of POSIX that are realistically usable on Linux and Solaris and maybe some obscure systems (you can probably do the same thing on Windows NT), notably this does not work on essentially anything BSD-derived.