linux-ext4 - Re: [PATCH] ext4: add noorlov parameter to avoid spreading of directory inodes

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Message-ID: <20131002165210.GA16076@kvack.org>
Date:	Wed, 2 Oct 2013 12:52:10 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: add noorlov parameter to avoid spreading of directory inodes

On Wed, Oct 02, 2013 at 06:44:56PM +0200, Lukáš Czerner wrote:
> What is the frequency of unlink operation in comparison to file
> creation ? There is a possible issue with the global goal cursors
> s_mb_last_group and s_mb_last_start which might make your files
> increasingly scattered across the disk. I've attempted to address
> this problem with my patch

unlink() is 1:1 with creation.  The storage on disk is essentially used 
as an elastic buffer for the rest of the system.  That said, unlink() may 
not occur for minutes, hours or days.

In terms of actual allocation pattern on disk, using fallocate() is 
sufficient to ensure that files on disk are usually limited to 1 extent, 
and allocated immediately following each other.  Without fallocate(), I 
was seeing allocations aligned to 2048 block boundaries for 9MB files, 
which hurt performance quite a bit.

> ext4: Try to better reuse recently freed space
> 
> What is the usual size of the files this application is creating ?

It varies.  The target is 8MB, but it ranges from 5MB to 22MB.  In the 
worst case it can be as small as 4KB.

		-ben

> Thanks!
> -Lukas

-- 
"Thought is the essence of where you are now."
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