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SubjectRe: [RFC 12/26] ext2 white-out support
In message <1185997941.18007.30.camel@kleikamp.austin.ibm.com>, Dave Kleikamp writes:
> On Wed, 2007-08-01 at 15:33 -0400, Josef Sipek wrote:
> > On Wed, Aug 01, 2007 at 02:10:31PM -0500, Dave Kleikamp wrote:
> > > On Wed, 2007-08-01 at 14:44 -0400, Josef Sipek wrote:
> > > > Now what? How do you rename? Do you rename in the same branch (assuming it
> > > > is rw)?
> > >
> > > Er, no. According to Documentation/filesystems/union-mounts.txt, "only
> > > the topmost layer of the mount stack can be altered".
> >
> > This brings up an very interesting (but painful) question...which makes more
> > sense? Allowing the modifications in only the top-most branch, or any branch
> > (given the user allows it at mount-time)?
>
> Your examples point out the complexity of trying to allow modifications
> at lower levels. It seems to me to be simpler (even if recursive copies
> are needed) to leave it as proposed.
[...]

There are three other reasons why Unionfs and our users like to have
multiple writable branches:

1. If only the topmost layer is writable, then every little change tends to
cause a copyup, which tends to clutter the top layer more quickly. Some
of our users didn't like that idea, while others explicitly wanted it --
so we give them a choice to decide, on a per layer/branch whether it
should be writable or readonly.

2. Some users unify different packages together. Imagine you union under
/union, several installed packages: /X11R6/{bin,man,lib,conf},
/apache/{bin,man,lib,etc}, and /mysql/{bin,man,lib,etc}, and so on. If a
user modifies /union/apache/etc/apache.conf, they sometimes want
apache.conf to remain in the writable branch it came from, not copied up.
That way all apache related files are logically left where they came
from, which makes administration easier. Again, some users like to have
multiple writable branches, and some don't -- so in Unionfs we give them
the choice. And yes, it does make our implementation more complex.

3. Some people use Unionfs in the scenario described in point #2 above, as a
poor man's space- and load- distribution system. Some of our users like
the idea of controlling how much storage space they give each branch, and
how much it might grow, and even how much CPU or I/O load might be placed
on each of the lower filesystems which serve a given branch. That way
they worry less about the top-layer's space filling up more quickly than
expected. Now Unionfs was never designed to be a load-balancing f/s (we
have RAIF for that, see <http://www.filesystems.org/project-raif.html>),
but users seems to always find creative ways to [ab]use one's software in
ways one never thought of. :-)

BTW, does Union Mounts copyup on meta-data changes (e.g., chmod, chgrp,
etc.)?

Erez.
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