Subject: Re: ext3 reservation question.
From: Chris Mason <mason@suse.com>
To: Andrew Morton <akpm@osdl.org>
Cc: Linus Torvalds <torvalds@osdl.org>, pbadari@us.ibm.com,
       linux-kernel@vger.kernel.org, ext2-devel@lists.sourceforge.net
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References: <200404211655.47329.pbadari@us.ibm.com>
	 <Pine.LNX.4.58.0404211959560.18945@ppc970.osdl.org>
	 <20040421204036.4e530732.akpm@osdl.org>
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Date: Thu, 22 Apr 2004 09:43:07 -0400
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On Wed, 2004-04-21 at 23:40, Andrew Morton wrote:

> The code I had at the time would reserve space in the filesystem
> correspnding to the worst-case occupancy based on file offset.  When we
> actually hit ENOSPC in prepare_write(), we force writeout, which results in
> those worst-space reservations being collapsed into their _real_ space
> usage, which is much less.  So writeout reclaims space in the filesystem
> and prepare_write() can proceed.
> 
> That worked fine on ext2.  But on ext3 we have a transaction open in
> prepare_write(), and the forced writeback will cause arbitrary amounts of
> unexpected metadata to be pumped into the current transaction, causing the
> fs to explode.
> 
> At least, I _think_ that was the problem.  All is hazy.
> 
One possible solution is to allocate holes in the file during
prepare_write/commit_write, logging the metadata as you go.  Then during
each commit fill any delayed allocations.  You've still got a
potentially unbounded operation for logging the bitmaps, maybe solvable
through creative reservations.

-chris

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