Re: Suggestion needed for fixing RAID6

[MRK <mrk@shiftmail.org>]

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]