From: "Jörn Engel" <joern@logfs.org>
To: Frantisek Rysanek <Frantisek.Rysanek@post.cz>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [noob q. on block layer] block IO read-ahead during sequential *write*?
Date: Mon, 7 Jan 2008 13:49:52 +0100 [thread overview]
Message-ID: <20080107124952.GA8201@lazybastard.org> (raw)
In-Reply-To: <47822835.4182.1ADF8AE7@localhost>
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
next prev parent reply other threads:[~2008-01-07 12:55 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-01-07 12:25 [noob q. on block layer] block IO read-ahead during sequential *write*? Frantisek Rysanek
2008-01-07 12:49 ` Jörn Engel [this message]
2008-01-07 15:51 ` Frantisek Rysanek
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