dCache uses pnfs as a filesystem and for storing meta-data. pnfs is a filesystem not designed for storage of actual files. Instead, pnfs manages the filesystem hierarchy and standard meta-data of a UNIX filesystem. In addition, other applications (as for example dCache) can use it to store their meta-data. pnfs keeps the complete information in a database.

pnfs implements an NFS server. All the meta-data can be accessed with a standard NFS client, like the one in the Linux kernel. After mounting, normal filesystem operations work fine. However, IO operations on the actual files in the pnfs will normally result in an error.

As a minimum, the pnfs filesystem needs to be mounted only by the server running the dCache core services. In fact, the pnfs server has to run on the same system. For details see (has to be written).

The pnfs filesystem may also be mounted by clients. This should be done by

[root] # mount -o intr,hard,rw <pnfs-server>:/pnfs /pnfs/<site.de>

(assuming the system is configured as described in the installation instructions). Users may then access the meta-data with regular filesystem operations, like ls -l, and by the pnfs-specific operations described in the following sections. The files themselves may then be accessed with the dCap protocol (see dCache Book Client Access and Protocols).

Mounting the pnfs filesystem is not necessary for client access to the dCache system if URLs are used to refer to files. In the grid context this is the preferred usage.