Re: raid5 failed while rebuiling - classical problem

[Roman Mamedov <roman@rm.pp.ru>]

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On Fri, 2 Jul 2010 23:18:54 +0200
Janek Kozicki <janek_listy@wp.pl> wrote:

> I need to overwrite the bad sector with zeros.

There isn't really a need to overwrite the whole drive for that -- you can
either get that block's address from the LBA number in dmesg errors, or by
running a read-only badblocks test. Then make sure it is really the sector you
want (try reading it with dd -- it will fail with an I/O Error), and overwrite
using dd if=/dev/zero of=/dev/hda seek=NNNNNNNNN bs=512 count=1
Several times if needed, checking if the drive has remapped the sector already
(instantly, or after some repeats it should become readable).

> I have a free hdc3, which I am going to use for that:
> 
>   # dd bs=512 if=/dev/hda3 of=/dev/hdc3 conv=noerror,sync
> 
> The noerror tells dd to continue, even if it cannot read, and sync
> tells to write zeros on hdc3 in that place (at least I hope that I
> got the manpage right).

There is the dd_rescue program, made specifically for that purpose:
http://www.garloff.de/kurt/linux/ddrescue/

> Then I will have an almost exact copy of bad hda3 on hdc3. 
> <...> maybe I could just --add hdc3 in place of hda3 ?

Yes, you could, and that would be the most logical thing to do (as hda might
continue failing and developing more bad sectors).

-- 
With respect,
Roman

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