linux-ext4 - Re: Bug in delayed allocation: really bad block layouts!

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Message-ID: <489F2B38.3060104@redhat.com>
Date:	Sun, 10 Aug 2008 12:54:00 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
CC:	linux-ext4@...r.kernel.org
Subject: Re: Bug in delayed allocation: really bad block layouts!

Theodore Ts'o wrote:
> Thanks to a comment on a recent blog entry of mine[1], I think I've
> uncovered a rather embarassing bug in mballoc.
> 
> [1]http://thunk.org/tytso/blog/2008/08/08/fast-ext4-fsck-times
> 
> I created a fresh 5 gig ext4 filesystem, and then populating it using a
> single-threaded tar command:
> 
> 	(cd /usr ; tar cf - bin lib) | (cd /mnt; tar xfp -)
> 
> I then unmounted the filesystem, and ran an instrumented e2fsck looking
> for fragmented files, and found a whole series of fragmanted files with
> the following pattern:

Hm, last year I reported "delalloc fragmenting files?" and Alex sent
some patches, I wonder if they all got integrated.  I guess I should
re-run that simple large-file dd test and see how we're doing now...

ok, on a 1G single-threaded write it looks sane:

Discontinuity: Block 53248 is at 100352 (was 98303)
Discontinuity: Block 116736 is at 165888 (was 163839)
Discontinuity: Block 180224 is at 231424 (was 229375)
Discontinuity: Block 243712 is at 296960 (was 294911)

so I suppose this is not the same issue ...

Hm, and I tried writing out 10 files in order as a simple test but
umount/remount brought me back many 0-byte files, I need to update my
patchset I guess.  :)

-Eric
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