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Date: Fri, 30 May 2008 03:31:49 -0400
From: Christoph Hellwig <hch@infradead.org>
To: Rik van Riel <riel@redhat.com>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Xiaoming Li <forrubm2@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [help]How to block new write in a "Thin Provisioning" logical
	volume manager as a virtual device driver when physical spaces run
	out?
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On Fri, May 30, 2008 at 09:04:41AM +1000, Dave Chinner wrote:
> > In a block based thin provisioning system, like you proposed, there
> > is no way to free up space.  Once a user's filesystem has written a
> > block, it is allocated - when the user deletes a file inside the
> > filesystem, the space will not be freed again...
> 
> That's where we've been discussing bio hints to help communicate
> space being allocated and freed by the filesystem to the lower layers.

Or use the XFS multiple subvolume support based on that:

http://verein.lst.de/~hch/xfs/xfs-multiple-containers.txt

and yeah, I really need to finish the lose bits up and actually post it.
Hopefully next month.