In PVFS1 there were two types of server processes, mgrs that served metadata and iods that served data. For any given PVFS1 file system there was exactly one active mgr serving metadata and potentially many iods serving data for that file system. Since mgrs and iods are just UNIX processes, some users found it convenient to run both a mgr and an iod on the same node to conserve hardware resources.
In PVFS2 there is exactly one type of server process, the pvfs2-server. This is also a UNIX process, so one could run more than one of these on the same node if desired (although we will not discuss this here). A configuration file tells each pvfs2-server what its role will be as part of the parallel file system. There are two possible roles, metadata server and data server, and any given pvfs2-server can fill one or both of these roles.
PVFS2 servers store data for the parallel file system locally. The current implementation of this storage relies on UNIX files to hold file data and a Berkeley DB database to hold things like metadata. The specifics of this storage are hidden under an API that we call Trove.