linux-kernel - Re: [PATCH] ext4: do not normalize block requests from fallocate.

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Message-ID: <20110523011117.GE10009@thunk.org>
Date:	Sun, 22 May 2011 21:11:18 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Vivek Haldar <haldar@...gle.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: do not normalize block requests from fallocate.

On Fri, May 13, 2011 at 02:19:05PM -0700, Vivek Haldar wrote:
> Currently, an fallocate request of size slightly larger than a power of
> 2 is turned into two block requests, each a power of 2, with the extra
> blocks pre-allocated for future use. When an application calls
> fallocate, it already has an idea about how large the file may grow so
> there is usually little benefit to reserve extra blocks on the
> preallocation list. This reduces disk fragmentation.
> 
> Tested: fsstress. Also verified manually that fallocat'ed files are
> contiguously laid out with this change (whereas without it they begin at
> power-of-2 boundaries, leaving blocks in between). CPU usage of
> fallocate is not appreciably higher. In a tight fallocate loop, CPU
> usage hovers between 5%-8% with this change, and 5%-7% without it.
> 
> Signed-off-by: Vivek Haldar <haldar@...gle.com>

Applied, with a minor change to avoid a bit assignment conflict with
the punch patches.

Also, I changed the commit message to note that using a file aging
simulator which filled the file system to 70%, the percentage of free
extents greater than 8MB (as measured using e2freefrag) increased from
38.8% without this commit, to 69.4% with this commit.

      	      	   	      	    	      - Ted
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