linux-kernel - Re: [RFC 12/26] ext2 white-out support

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Message-ID: <20070802120524.GY5101@hasse.suse.de>
Date:	Thu, 2 Aug 2007 14:05:24 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Josef Sipek <jsipek@....cs.sunysb.edu>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>, hch@...radead.org
Subject: Re: [RFC 12/26] ext2 white-out support

On Wed, Aug 01, Erez Zadok wrote:

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

...

>    And yes, it does make our implementation more complex.

And error-prone and unflexible wrt to changes. When XIP was introduced,
unionfs crashed all over this changes. I don't know if this has changed
yet. Not speaking of other issues like calling back into VFS (stack usage),
locking problems and so on.

> 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. :-)

And this has nothing to do with unioning ...

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

No. But it was proposed during on of the last postings.
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