linux-ext4 - Re: [PATCH] e4defrag: fallocate donor file only once

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Date:	Wed, 02 Sep 2009 16:24:04 -0600
From:	Andreas Dilger <adilger@....com>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Peng Tao <bergwolf@...il.com>, linux-ext4@...r.kernel.org,
	Theodore Tso <tytso@....edu>,
	Akira Fujita <a-fujita@...jp.nec.com>
Subject: Re: [PATCH] e4defrag: fallocate donor file only once

On Sep 02, 2009  18:09 -0400, Greg Freemyer wrote:
> On Wed, Sep 2, 2009 at 11:35 AM, Peng Tao<bergwolf@...il.com> wrote:
> > If we allocate the donor file once for all, it will have a better chance
> > to be continuous.
> >
> > Signed-off-by: "Peng Tao" <bergwolf@...il.com>
> 
> Seems like an improvement, but I'm not seeing any special handling for
> sparse files.  (Not before or after this patch.)
> 
> Seems like there should be an outer loop that identifies contiguous
> data block sets in a sparse file and defrags them individually as
> opposed to trying to defrag the entire file at once.
> 
> My impression is that with a large sparse file, e4defrag currently
> (with or without this patch) would fallocate a full non-sparse donor
> set of blocks the full size of the original file, then swap in just
> the truly allocated blocks?
> 
> If so, that is not very optimum.

And of course FIEMAP can be used to easily determine if the file is sparse.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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