nuttx/Documentation/components/filesystem/unionfs.rst

=================
Union File System
=================

Overview
========

This directory contains the NuttX Union File System.  The Union file
system is provides a mechanism to overlay two different, mounted file
systems so that they appear as one.  In general this works like this:

  1) Mount file system 1 at some location, say ``/mnt/file1``
  2) Mount file system 2 at some location, say ``/mnt/file2``
  3) Call ``mount()`` to combine and overly ``/mnt/file1`` and ``mnt/file2``
     as a new mount point, say ``/mnt/unionfs``.

``/mnt/file1`` and ``/mnt/file2`` will disappear and be replaced by the single
mountpoint ``/mnt/unionfs``.  The previous contents under ``/mnt/file1`` and
``/mnt/file2`` will appear merged under ``/mnt/unionfs``. Files at the same
relative path in file system1 will take presence. If another file of the
same name and same relative location exists 	in file system 2, it will
not be visible because it will be occluded by the file in file system1.

See ``include/nutts/unionfs.h`` for additional information.

The Union File System is enabled by selecting the ``CONFIG_FS_UNIONFS`` option
in the NuttX configuration file.

Disclaimer:  This Union File System was certainly inspired by UnionFS
(http://en.wikipedia.org/wiki/UnionFS) and the similarity in naming is
unavoidable.  However, other than that, the NuttX Union File System
has no relationship with the UnioinFS project in specification, usage,
design, or implementation.

Uses of the Union File System
==============================

The original motivation for this file was for the use of the built-in
function file system (BINFS) with a web server.  In that case, the built
in functions provide CGI programs.  But the BINFS file system cannot hold
content.  Fixed content would need to be retained in a more standard file
system such as ROMFS.  With this Union File System, you can overly the
BINFS mountpoint on the ROMFS mountpoint, providing a single directory
that appears to contain the executables from the BINFS file system along
with the web content from the ROMFS file system.

Another possible use for the Union File System could be to augment or
replace files in a FLASH file system.  For example, suppose that you have
a product that ships with content in a ROMFS file system provided by the
on-board FLASH.  Later, you overlay that ROMFS file system with additional
files from an SD card by using the Union File System to overlay, and
perhaps replace, the ROMFS files.

Another use case might be to overlay a read-only file system like ROMFS
with a writable file system (like a RAM disk).  This should then give
to a readable/write-able file system with some fixed content.

Prefixes
========

And optional prefix may be provided with each of the file systems
combined in by the Union File System.  For example, suppose that:

* File system 1 is a ROMFS file system with prefix == NULL,
* File system 2 is a BINFS file system with prefix == "cgin-bin", and
* The union file system is mounted at ``/mnt/www``.

Then the content in the in the ROMFS file system would appear at
``/mnt/www`` and the content of the BINFS file system would appear at
``/mnt/www/cgi-gin``.

Example Configurations
======================

* ``boards/sim/sim/sim/unionfs`` - This is a simulator configuration that
  uses the Union File System test at apps/examples/unionfs.  That test
  overlays two small ROMFS file systems with many conflicts in
  directories and file names.  This is a good platform for testing the
  Union file System and apps/examples/unionfs is a good example of how to
  configure the Union File System.

* ``boards/arm/lpc17xx_40xx/lincoln60/thttpd-binfs`` - This is an example
  using the THTTPD web server.  It server up content from a Union File
  System with fixed content provided by a ROMFS file system and CGI
  content provided by a BINFS file system.

  You can see how the Union File System content directory is configured
  by logic in apps/example/thttpd/.

* ``boards/arm/lpc17xx_40xx/olimex-lpc1766stk/thttpd-binfs`` - This is
  essentially the same as the lincoln60 configuration.  It does not work,
  however, because the LPC1766 has insufficient RAM to support the THTTPD
  application in this configuration.

See the README.txt file in each of these board directories for additional
information about these configurations.