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NFS File System Model and the Mount Protocol
(Page 2 of 3)
The Mount Protocol
Before NFS can be used to allow a
client to access a file on a remote server, the client must be given
a way of accessing the file. This means that a portion of the remote
file system must be made available to the client, and the file opened
for access. A specific decision was made when NFS was created to not
put file access, opening and closing functions into NFS proper. Instead,
a separate protocol was created to work with NFS, so that if in the
future the method of providing file access needed to be changed, it
wouldn't require changes to NFS itself. This separate mechanism is called
the Mount Protocol, and is described in Appendix A of RFC 1094
(NFSv2). Note that while functionally distinct, Mount is considered
part of the overall NFS package.
When NFS was revised to version 3,
the Mount Protocol was similarly modified. The NFSv3 version of the
Mount Protocol is defined in Appendix I of RFC 1813 (NFSv3). It contains
some changes to how the protocol works, but the overall operation of
the two versions of Mount is pretty much the same.
The term “mount” is actually
an analog to a hardware term that refers to making a physical storage
volume available. In the “olden dayse” storage devices were
usually removable disk packs, and to use one you mounted it onto a drive
unit. In a similar manner, NFS resources are logically mounted using
the Mount protocol, which makes the shared file system available to
the client. A file can then be opened and a file handle returned to
the NFS client so it can reference the file for operations such as reading
and writing.
 Key Concept: Versions 2 and 3 of NFS do not include procedures for opening or closing resources on a remote server. Before NFS tasks can be accomplished on these versions, the special Mount protocol must be employed to mount a filesystem and create a file handle to access a file on it; the protocol is also used to unmount the file system when no longer required. The Mount protocol is implemented in a manner very similar to NFS itself, defining procedures that use RPC and XDR. In version 4 of NFS, Mount is no longer needed; the tasks it performs are implemented as NFSv4 operations.
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