linux-ext4 - Re: ext4

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Date:	Tue, 03 Jul 2012 21:15:16 -0400
From:	Phillip Susi <psusi@...ntu.com>
To:	Jan Kara <jack@...e.cz>
CC:	Eric Sandeen <sandeen@...hat.com>, Theodore Ts'o <tytso@....edu>,
	Fredrick <fjohnber@...o.com>, Ric Wheeler <rwheeler@...hat.com>,
	linux-ext4@...r.kernel.org, Andreas Dilger <adilger@...ger.ca>,
	wenqing.lz@...bao.com
Subject: Re: ext4_fallocate

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On 07/02/2012 01:44 PM, Jan Kara wrote:
>   Yes, that option is broken and basically unfixable for data=ordered mode
> (see http://comments.gmane.org/gmane.comp.file-systems.ext4/30727). For
> data=writeback it works fine AFAICT.

Does data=writeback with barriers fix this fallocate problem?

I can see that writing small random bits into a very large uninitialized file would cause a lot of updates to the extent tree, but with a sufficiently large cache, shouldn't many of the small, random writes become coalesced into fewer, larger extent updates that are done at delayed flush time?  Are you sure that the extent tree updates are not being done right away and blocking the writing application, instead of being delayed?


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