Re: [PATCH 2.6.27.y 2/3] ext4: Fix file fragmentation during large file write.

["Jayson R. King" <dev@jaysonking.com>]

On 05/28/2010 08:06 PM, tytso@mit.edu wrote:
> On Fri, May 28, 2010 at 02:26:57PM -0500, Jayson R. King wrote:
>> From: Aneesh Kumar K.V<aneesh.kumar@linux.vnet.ibm.com>
>> Date: Thu Oct 16 10:10:36 2008 -0400
>> Subject: ext4: Fix file fragmentation during large file write.
>>
>> commit 22208dedbd7626e5fc4339c417f8d24cc21f79d7 upstream.
>>
>> The range_cyclic writeback mode uses the address_space writeback_index
>> as the start index for writeback.  With delayed allocation we were
>> updating writeback_index wrongly resulting in highly fragmented file.
>> This patch reduces the number of extents reduced from 4000 to 27 for a
>> 3GB file.
>
> This isn't a critical bug fix either.  I don't really care a whole
> lot, since I don't plan to support ext4 with all of these patches but
> if you haven't been doing a full set of testing with these patches,
> I'd be very concerned about whether ext4 would be stable after
> applying this patch series.
>
> What sort of testing _have_ you done?

I've ran dbench for hours on an ext4 volume followed by fsck on the 
volume. Without the patches (particularly, just the last patch, "ext4: 
Implement range_cyclic..."), a typical, sustained moderate to high ext4 
fs load on .27 would often lead to a deadlock. A good demonstration is 
to run "dbench 500" which will usually cause a deadlock in a couple of 
minutes.

I wasn't aware of a way to apply the deadlock fix in "ext4: Implement 
range_cyclic..." without also introducing this patch, since some of the 
blocks it touches are created by this patch.

Thanks for looking.

Rgds,

Jayson