Copy-on-write NFS server
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Copy-on-write NFS server

This is a copy-on-write NFS server designed to allow dynamic stacking of filesystem hierarchies. In a typical use, a user will mount a read-only base image (such as a standard Linux distribution installation) and a private, writable overlay. Initially, the stacked filesystem will appear to be the same as the base image, but any changes made are captured in the private overlay, allowing the user to get the benefits of a private filesystem without having to start from scratch.

This was designed for use with the XenoServers public computing platform, particularly to allow multiple virtual machines using the Xen hypervisor to run independently while sharing a common base filesystem. This makes it easy and fast to launch a new virtual machine from an existing filesystem without worrying about write conflicts.

The source code is written in Objective Caml (OCaml), which can be obtained from:

or it is available with most Linux distributions. To build it, use make. First change OCAMLINCLUDE in the Makefile to point to your OCaml library directory. To build a native executable (the default) use:

    make opt

To build a bytecode executable use:

    make bc

In each case, the executable cownfsd is built, which can then be copied to any directory (there are no other dependencies in the build tree).

This daemon uses the standard C rpcgen and its accompanying libraries and generates code to interface between the RPC code and the OCaml code. This generator is derived from the ocamlrpcgen library written by Gerd Stolpmann. Unfortunately, ocamlrpcgen was a huge CPU hog that destroyed peformance and crashed regularly, so I don't use the library itself, but I did extend the code generator to produce my interface code. The ocamlrpcgen library is also distributed under the GPL and can be found here:

This code is Copyright 2004, 2005 by Russ Ross and the latest release is available at the site referenced above. It is distributed under the GPL; full details are available in the file COPYING.

I can be contacted by the email address: russ at russross dot com.

Note: this software was written for research purposes and has not been designed with rigorous security in mind. It should only be used in trusted environments.


I wrote a copy-on-write NFS daemon for use with the XenoServers project. It's a general purpose userspace NFS daemon with the ability to dynamically stack file hierarchies over each other. Different layers can be configured as writable or as read-only, and in the latter case changes are made to a writable overlay in a copy-on-write fashion.

Some more information is available in a paper from WORLDS'04 (PDF format).


A simple example illustrates the basic idea.

  • Install a pristine Linux kernel source tree in /usr/src/linux

  • Create an empty directory /home/russ/testpatches

  • Start cownfsd with /home/russ/testpatches as the root exported directory and mount it at /mnt/cow

  • Note: the path you give to mount is relative to the path you specify to cownfsd, so in this example I would have issued these two commands, where localhost:/ refers to the root exported directory (/home/russ/testpatches) and /mnt/cow is the mount point:

    1. cownfsd /home/russ/testpatches &
    2. mount -t nfs localhost:/ /mnt/cow
  • Create a subdirectory /home/russ/testpatches/linux1

  • Create a file called .mount in the linux1 directory with the single line:

    read "/usr/src/linux"

  • The directory /mnt/cow/linux1 is immediately populated with the complete contents of the /usr/src/linux directory.

  • Make changes within /mnt/cow/linux1, say running make config

  • All new and modified files are updated in a sparsely created directory structure rooted at /home/russ/testpatches/linux1. A deleted file foo is marked by the creation of an empty file called .~~foo

Similarly, you could create a new directory called /home/russ/testpatches/linux2 which would be visible as /mnt/cow/linux2 and could mount the same read-only base image (/usr/src/linux). Changes made in the two directories are completely isolated from each other.

This was written originally for use with Xen. Multiple domains can use overlays to create custom versions of a common root filesystem, which they can then use for booting. This lets multiple concurrent domains share the same root image by booting over NFS with customized overlays.

Basic rules

  • Mounts can be many levels deep, and each layer can be read-only or writable
  • The top layer must always be writable (though file permissions may still prevent writes from succeeding), and files in higher levels mask files in lower levels.
  • A file called .~~name in a higher level hides a file called name in a lower level.
  • Every file is visible from exactly one layer (copy-on-write works at the whole-file level)
  • If the visible instance of a file is in a writeable layer, then all writes are made directly to it. If the visible layer is readonly, then the first write to the file triggers a copy to the top level which then becomes the visible instance of the file.
  • New files are always created in the top writable directory
  • Entries in .mount files are one per line. A .mount file can exist in any directory, and affects all subdirectories recursively.
  • .mount file entries can be one of three types: *# read "/readonly/path" *# write "/writeable/path" *# replace "/writable/path"
  • Later entries in a .mount file are layered on top of earlier entries. The directory containing the .mount file is always layered on top as writeable, except when the last entry uses replace.

Common problems

The most common problem people have reported to me is a failure to mount properly. The main symptom is a bunch of errors having to do with stale file handles showing up either at mount time or immediately after.

The solution to this problem is usually to kill any other mountd processes running on the same machine. cownfsd provides its own Mount service, and the NFS service only works with it.

Mount and NFS services are closely tied together (NFSv4 combines them into a single service). The Mount service returns a file handle identifying the root directory of the mount point, which is later passed to the NFS service as a reference point for future operations. The structure of the file handle is unique to a particular NFS implementation so the services responding to Mount and NFS requests must be matched. In particular, the standard kernel NFS implementation uses a file's inode number as the main part of its file handle, which doesn't really work with a userspace daemon like cownfsd.

The version problem also manifests itself when booting over an NFS-mounted file system (a very useful thing to do with cownfsd in a Xen environment). If you are planning to boot Linux from a cownfsd volume, be sure to add ,v3 to the end of the nfsroot line in your boot parameter list.

Getting CoWNFSd

The server is written in OCaml and the source is available under the GPL. It is currently housed on github, after being converted from an old Bazaar/Arch repository.


Russ Ross (russ at russross dot com)


You can download this project in either zip or tar formats.

You can also clone the project with Git by running:

$ git clone git://