linux-ext4 - Re: ext4

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Date:	Thu, 28 Jun 2012 11:23:14 -0400
From:	Eric Sandeen <sandeen@...hat.com>
To:	Phillip Susi <psusi@...ntu.com>
CC:	Andreas Dilger <adilger@...ger.ca>, Fredrick <fjohnber@...o.com>,
	linux-ext4@...r.kernel.org
Subject: Re: ext4_fallocate

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On 6/28/12 11:12 AM, Phillip Susi wrote:
> On 6/25/2012 3:33 AM, Andreas Dilger wrote:
>> There was a recent patch series "ext4: add an io-tree to track
>> block allocation" that may improve the performance for your case of
>> overwrite of uninitialized files, but it hasn't landed yet.
> 
> I'm confused.  Why is writing to uninitialized extents slow, and why
> would this help?  If you have an uninitialized extent, then the blocks
> are already allocated, just flagged as containing uninitialized data.
>  Writing to them should be no different than writing to initialized
> extents, save for the step of clearing the uninitialized flag.

The other piece is that large uninitialized extents get split into
up to 3 extents if you write into the middle of them so there are
potentially a lot more metadata updates flying around.

- -Eric
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