Re: Union-FS options

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On Wed, 2007-01-17 at 22:26 +0530, Saurabh Jain (सौरभ जैन) wrote:
> Hi!
> 
> I create read-only "thin client images" and diskless boot setups quite
> frequently. I have been studying UnionFS quite closely myself. I had a
> choice between UnionFS, UnionFSFuse and FunionFS. I chose to try
> UnionFS first. Though it is reported to be broken and badly
> implemented, among the three implementations this is the one with the
> maximum number of users.
> 
> I am happy to say that after a month of testing, UnionFS works
> perfectly and hasn't given a squeak. I used kernel 2.6.18 on a Fedora
> Core 6 system.
> 
> Maybe it's time to give it another look. It is such a welcome break
> from watching all those mount/umount errors while bootup/shutdown.
I'm not terribly inclined to revisit unionfs until it's a part
of the standard kernel.  Plus, for our needs a _simple_ user
space implementation should be sufficient.  And the FUSE 
implementations are very simple.

[ Also note, I read somewhere that the authors of unionfs are
  working on a totally new implementation, so the original code
  base _may_ get dropped in favor of the new implementation
  at some point. ]


unionfs-fuse isn't suitable for our needs -- I haven't been able
to convince it to layer a rw directory on top of a ro directory.
It truly just unions the filesystems and provides little else.

funionfs is a little more attractive.  We'd do something like
mount --bind /etc /.etc
funionfs -o dirs=/.etc=RO:/var/lib/stateless/state/etc /etc -o nonempty

That would give us "writable" /etc where all the writes are
redirected into /var/lib/stateless/state/etc.  The odd /.etc is
necessary as the mountpoint can not be the same directory as the
ro or rw directories.

Once we have a writable /etc we also need logging of write operations
so that we know if something changed that shouldn't have changed.  That
should be fairly easy to add to funionfs (using the log fuse filesystem
as a template).

Jeff


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