Vladimir Kolpakov


Funifs - FUSE read-only Union Filesystem


Funifs implements a limited set of unionfs filesystem features (a read-only union of read-only branches) sufficient for staging web site testing and development.

The primary motivation for funifs comes from the KISS principle: make something easy, stable and extensionsable.




After downloading and unpacking the tarball, run this in the directory with Build.PL:

./Build.PL ; ./Build ; ./Build test ; ./Build install


 funifs  - /usr/sbin/funifs - funifs userspace FUSE driver
 rcfunifs - /etc/init.d/funifs - service script to mount/unmount at system boot
 fstab (in /usr/share/doc/packages/funifs/) - examples of /etc/fstab lines.


For those of you who are as paranoid as me, before installing to your system directories, make sure:

./Build.PL --install_base=/var/tmp/Funifs ; ./Build ; ./Build test ; ./Build install

If running it as a regular user fails, add "user_allow_other" to /etc/fuse.conf. That should make the Build test happy. Or, just run it as root.


Let's say we have a production website configured like this:

  ServerName      www.site.com
  DocumentRoot    /srv/site.com/www/root

And a staging website configured like this:

  ServerName      w.u.site.com
  DocumentRoot    /srv/site.com/u/w/root

To mount the staging directory tree (w.u) on top of the production directory tree (www), configure /etc/fstab like this:

 /u/w /srv/site.com/u/w fuse.funifs dirs=/delta/site.com/u/w:/srv/site.com/www 0 0

Your staging website's DocumentRoot now consists of the production tree overlain with any differences from the staging tree. Delta branches sandwich can have multiple layers, not just one.


Funifs relies on FUSE library, fuse.ko kernel module, and perl modules:

 * Fuse
 * Filesys::Statvfs
 * Unix::Syslog
 * Module::Build

Perl module Fuse.pm works flawless starting version Fuse-0.08. Multi-threaded use has buggy history on RHEL/Centos 5.


It's recommended to disable filesystem crawling for "fuse" type filesystems. You may want to update system configuration files, such as /etc/sysconfig/locate /etc/cron.daily/slocate.cron /etc/updatedb.conf /usr/share/msec/security.sh


Funifs is not a comprehensive driver, it has limited features due to specific demands and stacking nature. The most visible are:

Lack of whiteouts support

"Whiteout" is the way to "remove" file from the union, when it physically exists in one of branches. That is, if program code relies on a file presence test

-f $path ...

and file exists in bottom branch of union, funifs has no way to emulate removal by manipulating the delta branches content.

Application code should not rely on a file presence. For a templates (where empty file equivalent to missing file) the lack of whiteout-s is not an issue: zero-size file in top layer has the same effect as the whiteout.

Lack of persistent i-nodes support

Unionfs semantics by it's nature is inaccurate about files i-nodes, thus any attempt to compare two files i-nodes most likely will return false, even for identical files on underlying filesystem branches. Application code should not rely on i-node values.

Copy file to "itself" bug

Attempt to copy file to delta branch from another branch using the union as the source tends to produce empty file of non-zero length. Fix for this issue is not known: funifs driver is unable to control this. Application code should refer to bottom branch content explicitly, rather then use result of union-ing.


Copyright (C) Vladimir V. Kolpakov [aka double-you] 2006-2011

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.


To Stephane Saux and Ben Schein at SFGate for inspiration and insistency, to Mark Glines and Dobrica Pavlinusic for the bridge FUSE--Perl, to Denis Cirulis at opensource for the bugs hunting.


I would really appreciate it if you could publish your ideas, bug reports, or feature request on http://groups.google.com/group/funifs/topics or even submit fixes on funifs.googlecode.com.

Releasing new version on googlecode, don't forget also upload tarball to CPAN, that is result of the

./Build dist


Funion - somewhat similar to funifs, but seems unfinished http://code.google.com/p/funion/

Unionfs-fuse - read/write FUSE union http://podgorny.cz/moin/UnionFsFuse

Perl wrapper for FUSE http://search.cpan.org/~dpavlin/Fuse/