linux-ext4 - Re: delalloc fragmenting files?

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Date:	Wed, 7 Nov 2007 13:37:16 -0700
From:	Andreas Dilger <adilger@....com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Alex Tomas <bzzz.tomas@...il.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: delalloc fragmenting files?

On Nov 06, 2007  13:54 -0600, Eric Sandeen wrote:
> Hmm bad news is when I add uninit_groups into the mix, it goes a little
> south again, with some out-of-order extents.  Not the end of the world,
> but a little unexpected?
> 
> ....
> Discontinuity: Block 1430784 is at 24183810 (was 24181761)
> Discontinuity: Block 1461760 is at 24216578 (was 24214785)
> Discontinuity: Block 1492480 is at 37888 (was 24247297)
> Discontinuity: Block 1519616 is at 850944 (was 65023)
> Discontinuity: Block 1520640 is at 883712 (was 851967)
> Discontinuity: Block 1521664 is at 1670144 (was 884735)
> Discontinuity: Block 1522688 is at 2685952 (was 1671167)
> Discontinuity: Block 1523712 is at 4226048 (was 2686975)
> Discontinuity: Block 1524736 is at 11271168 (was 4227071)
> Discontinuity: Block 1525760 is at 23952384 (was 11272191)

I think part of the issue is that by default the groups marked BLOCK_UNINIT
are skipped, to avoid dirtying those groups if they have never been used
before.  This policy could be changed in the mballoc code pretty easily if
you think it is a net loss.  Note that the size of the extents is large
enough (120MB or more) that some small reordering is probably not going
to affect the performance in any meaningful way.

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

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