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

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Date:	Wed, 1 Oct 2008 07:22:12 -0500
From:	Milton Miller <miltonm@....com>
To:	"Janos Haar" <janos.haar@...center.hu>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: How can i read really 512 byte?


On Oct 1, 2008, at 5:23 AM, Janos Haar wrote:

>> 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
>
> Hello,
>
> Thank you for the information.
> Can you help me a little bit more?
> Where need to add this parameter?
> In the kernel, in the blocked's source, or in the dd_rescue's source?

dd_rescue ... there is a call to open, add O_DIRECT to the second 
argument and recompile.  If it says unknown identifier, just copy the 
#define from the kernel header into the dd_rescue source.

milton

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