linux-ext4 - Re: [PATCH v3 3/6] ext4: cache extent hole in extent status tree for ext4_da_map

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Date:	Thu, 4 Sep 2014 21:05:50 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH v3 3/6] ext4: cache extent hole in extent status tree for
 ext4_da_map_blocks()

On Wed, Aug 27, 2014 at 03:55:16PM +0200, Jan Kara wrote:
> On Thu 07-08-14 11:35:50, Zheng Liu wrote:
> > From: Zheng Liu <wenqing.lz@...bao.com>
> > 
> > Currently extent status tree doesn't cache extent hole when a write
> > looks up in extent tree to make sure whether a block has been allocated
> > or not.  In this case, we don't put extent hole in extent cache because
> > later this extent might be removed and a new delayed extent might be
> > added back.  But it will cause a defect when we do a lot of writes.
> > If we don't put extent hole in extent cache, the following writes also
> > need to access extent tree to look at whether or not a block has been
> > allocated.  It brings a cache miss.  This commit fixes this defect.
> > Meanwhile, if an inode has no any extent, this extent hole also will
> > be cached.
> > 
> > Cc: "Theodore Ts'o" <tytso@....edu>
> > Cc: Andreas Dilger <adilger.kernel@...ger.ca>
> > Cc: Jan Kara <jack@...e.cz>
> > Signed-off-by: Zheng Liu <wenqing.lz@...bao.com>
>   So I agree with this change, it certainly doesn't make things worse. But
> when looking into ext4_ext_put_gap_in_cache() I have one question: That
> function uses ext4_find_delalloc_range() to check that the intended range
> doesn't have any delalloc blocks in it. However it doesn't make any effort
> to put a smaller hole in cache - for example if we have blocks allocated
> like:
> 
> 5-6 = delalloc
> 7-10 = allocated
> 
> and ext4_ext_put_gap_in_cache() is called for block 0, the function won't
> put anything into cache although it could put range 0-4 in the cache as a
> hole. Why is that?

Oops, it should put range 0-4 in the cache.  Thanks for pointing it out.
I will fix it.

Regards,
                                                - Zheng
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