linux-ext4 - Re: [REGRESSION] allocated N with only M reserved metadata blocks

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Message-ID: <20130313084318.GA11553@gmail.com>
Date:	Wed, 13 Mar 2013 16:43:18 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	Eric Whitney <enwlinux@...il.com>, linux-ext4@...r.kernel.org,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [REGRESSION] allocated N with only M reserved metadata blocks

On Tue, Mar 12, 2013 at 12:19:13PM -0400, Theodore Ts'o wrote:
> On Tue, Mar 12, 2013 at 03:11:23PM +0100, Lukáš Czerner wrote:
> > So there is indeed a problem with the mentioned commit
> > 
> > 67a5da564f97f31c4054d358e00b34d7ee570da5
> > 
> > Due to the bug in that code is has exactly the opposite result -
> > with this commit we will _never_ zero out blocks instead of creating
> > uninitialized extents. In other words, we will always create
> > uninitialized extent.
> 
> Whoops.  I even remember how this bug happened.  Originally
> max_zeroout was in file system blocks, and it was suggested that we
> change this to use units of kilobytes instead.  Unfortunately, this
> change wasn't done completely.  :-(
> 
> > This can be easily fixed by the following patch (which makes the
> > warning go away), but it brings up a question whether this "optimization"
> > was worth it in the first place since noone noticed that it had exactly
> > the opposite effect than it should have had :)
> 
> Well, I had noticed that random AIO workloads resulted in the extent
> tree getting far more fragmented than I had expected.  (See previous
> discusisons about how we really need to improve our ability to merge
> empty leaf and index nodes in the extent tree.)

Agree, we could do better in merging extent tree.  I will take a look at
it, but, sorry, my plate is too full recently.  So it has a low priority.

Regards,
                                                - Zheng
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