linux-kernel - Re: [noob q. on block layer] block IO read-ahead during sequential *write*?

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Message-ID: <20080107124952.GA8201@lazybastard.org>
Date:	Mon, 7 Jan 2008 13:49:52 +0100
From:	Jörn Engel <joern@...fs.org>
To:	Frantisek Rysanek <Frantisek.Rysanek@...t.cz>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [noob q. on block layer] block IO read-ahead during sequential *write*?

On Mon, 7 January 2008 13:25:09 +0100, Frantisek Rysanek wrote:
> 
> let me start with a simple example. The following commands:
> 
>  cp /dev/zero /dev/hda
>  dd if=/dev/zero of=/dev/hda [bs=512]
> 
> both have one common side-effect: apart from the disk being properly 
> overwritten with zeroes, the kernel seems to keep reading sectors 
> ahead of the current seek position of the sequential write.

Block devices are cached in the page cache.  If you write less than a
full page, any remainder has to be read from the device.

If you retry the dd with bs=4096 (or whatever your architecture's page
size happens to be), does this still occur?

Jörn

-- 
Chance favors only the prepared mind.
-- Louis Pasteur
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