Re: Suggestion needed for fixing RAID6

["Janos Haar" <janos.haar@netcenter.hu>]

----- Original Message ----- 
From: "MRK" <mrk@shiftmail.org>
To: "Neil Brown" <neilb@suse.de>
Cc: "Janos Haar" <janos.haar@netcenter.hu>; <linux-raid@vger.kernel.org>
Sent: Monday, May 03, 2010 12:04 PM
Subject: Re: Suggestion needed for fixing RAID6


> On 05/03/2010 04:17 AM, Neil Brown wrote:
>> On Sat, 1 May 2010 23:44:04 +0200
>> "Janos Haar"<janos.haar@netcenter.hu>  wrote:
>>
>>
>>> The general problem is, i have one single-degraded RAID6 + 2 badblock 
>>> disk
>>> inside wich have bads in different location.
>>> The big question is how to keep the integrity or how to do the rebuild 
>>> by 2
>>> step instead of one continous?
>>>
>> Once you have the fix that has already been discussed in this thread, the
>> only other problem I can see with this situation is if attempts to write 
>> good
>> data over the read-errors results in a write-error which causes the 
>> device to
>> be evicted from the array.
>>
>> And I think you have reported getting write
>> errors.
>>
>
> His dmesg AFAIR has never reported any error of the kind "raid5:%s: read 
> error NOT corrected!! " (the error message you get on failed rewrite 
> AFAIU)
> Up to now (after my patch) he only tried with MD above DM-COW and DM was 
> dropping the drive on read error so I think MD didn't get any opportunity 
> to rewrite.
>
> It is not clear to me what kind of error MD got from DM:
>
> Apr 29 09:50:29 Clarus-gl2k10-2 kernel: device-mapper: snapshots: 
> Invalidating snapshot: Error reading/writing.
> Apr 29 09:50:29 Clarus-gl2k10-2 kernel: ata8: EH complete
> Apr 29 09:50:29 Clarus-gl2k10-2 kernel: raid5: Disk failure on dm-1, 
> disabling device.
>
> I don't understand from what place the md_error() is called...
> but also in this case it doesn't look like a rewrite error...
>
> I think without DM COW it should probably work in his case.
>
> Your new patch skips the rewriting and keeps the unreadable sectors, 
> right? So that the drive isn't dropped on rewrite...
>
>> The following patch should address this issue for you.
>> It is*not*  a general-purpose fix, but a specific fix
> [CUT]

Just a little note:
I have 2 bad drives, one wich have bads at 54%, have >2500 UNC sectors, wich 
is too much for trying it to repair, this drive is really failing....
The other have only 123 bads at 99% wich is a very small scratch on the 
platter, now i am trying to fix this drive instead.
The repair-check sync process now runs, i will reply soon again...

Thanks,
Janos