NSS has limited support for memory mapped files, primarily to support loading programs. NSS does not fully support memory mapped files especially if the application uses sparse files.
For example, the CopyCat application used by Netatalk uses sparse files for its database. Netatalk tries to create a CopyCat database as a sparse file called .AppleDB in the root of the volume by using memory mapped IO. This can cause the server to hang if you are using an NSS volume as the Netatalk share because of the limited support in NSS for this combination.