linux-ext4 - Re: delalloc fragmenting files?

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Message-ID: <4728ED3B.1060407@redhat.com>
Date:	Wed, 31 Oct 2007 16:01:47 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@....com>
CC:	ext4 development <linux-ext4@...r.kernel.org>,
	Alex Tomas <alex.tomas@....com>
Subject: Re: delalloc fragmenting files?

Andreas Dilger wrote:
> 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.

I've been testing this with a 16x1MB buffered dd to a fresh filesystem;
it gets 7 extents, and out of order.  :(

First block: 37120
Last block: 63427
Discontinuity: Block 1024 is at 38400 (was 38143)
Discontinuity: Block 1036 is at 57344 (was 38411)
Discontinuity: Block 2048 is at 38412 (was 58355)
Discontinuity: Block 2072 is at 61440 (was 38435)
Discontinuity: Block 3072 is at 38436 (was 62439)
Discontinuity: Block 3108 is at 62440 (was 38471)
/mnt/test/testfile: 7 extents found

One thing that seems to be happening is that thanks to delalloc, a nice
big request is coming in (only 1036 blocks of the 4096, not quite sure
why), but then it gets into ext4_mb_normalize_request(), which finds the
most blocks it can "preallocate" is 256, and chops down the request to
256 blocks.  Shouldn't this preallocation be over & above what was asked
for, vs. reducing the request?

Ok, so, we get allocations in 256-block chunks...  Why they don't all
come out contiguous, I don't know yet...

-Eric
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