Each of the Berkeley DB subsystems is described by one or more shared memory regions. These regions live in the environment home directory, and contain all of the shared information, including mutexes, that describes the Berkeley DB environment.
The Berkeley DB library uses the POSIX mmap (or other similar) interface to map the shared memory regions. Most remote file systems (e.g., the Network File System (NFS) and the Andrew File System (AFS)), do not support mapping files into process memory. For this reason, we strongly recommend that the database home directory reside in a local filesystem.
For remote file systems that do allow system files to be mapped into process memory, it is important to note that home directories accessed via remote file systems cannot be used simultaneously from multiple clients. None of the commercial remote file systems available today implement a coherent, distributed shared memory paradigm for remote-mounted files. As a result, different machines will see different versions of these shared regions and the system behavior is undefined.
Databases, log files and temporary files may be placed on remote filesystems, although the application may incur a performance penalty for doing so.