linux-kernel - How can i read really 512 byte?

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Date:	Tue, 30 Sep 2008 09:50:05 -0500 (CDT)
From:	Milton Miller <miltonm@....com>
To:	"Janos Haar" <janos.haar@...center.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: How can i read really 512 byte?

On Tue, 30 Sep 2008 at 08:29:15 -0400 (EDT), Janos Haar wrote:
> Now i am working on recover some data from one defective drive.  I
> am using dd_rescue, and dd, but both ready only 4K, i think, because
> the kernel's block size is 4K.
> 
> I have tried to google for the solution, but only found this trick:
> 
> losetup /dev/loop0 /dev/hdc
> blockdev --setbsz 512 /dev/hdc
> 
> and after this set, trying to read the hdc...
> 
> It looks like working on the first look, but not really. :-( The
> 512 size bad sectors still have 4K sizes, but the reading attempt
> takes more (8x ?) longer.
> 
> Somebody can help me to set the reading block size to 512 byte?

You need to add O_DIRECT to the open call in the flags parameter
(the second argument).   (My brother tested this a year or so ago,
and it worked for him).

milton
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