linux-ext4 - Re: delalloc fragmenting files?

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Message-ID: <20071026221938.GV3042@webber.adilger.int>
Date:	Fri, 26 Oct 2007 16:19:38 -0600
From:	Andreas Dilger <adilger@....com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Alex Tomas <alex.tomas@....com>
Subject: Re: delalloc fragmenting files?

On Oct 26, 2007  16:24 -0500, Eric Sandeen wrote:
> The resulting file had over 4k extents.
> [root@...r-05 ~]# filefrag -v /mnt/test/foobar  | grep -i extents
> File is stored in extents format
> /mnt/test/foobar: 4075 extents found

On a related note - we're just putting the finishing touches on the
FIEMAP patches for ext4 + e2fsprogs, so that we can get decent looking
output from filefrag, and much more efficiently than FIBMAP.

> if I don't mount with delalloc:
> 
> mount -t ext4dev -o data=writeback,extents,mballoc /dev/sdb7 /mnt/test
> 
> and run the same dd, I get 229 extents:
> 
> [root@...r-05 ~]# filefrag -v /mnt/test/foobar  | grep -i extents
> File is stored in extents format
> /mnt/test/foobar: 229 extents found

One of the issues is that w/o delalloc the mballoc code only gets 
single-block allocations, so there might be a problem with the interface
to mballoc.  That might be caused by the fact the patches were changed
at one point from delalloc-atop-mballoc to mballoc-atop-delalloc, and
something was missed in that conversion.

Have you tried O_DIRECT?  That is another way to access mballoc w/o
using delalloc.

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

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