Re: Suggestion needed for fixing RAID6

[Neil Brown <neilb@suse.de>]

On Mon, 03 May 2010 12:04:38 +0200
MRK <mrk@shiftmail.org> wrote:

> 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.

Hmmm... fair enough.

> 
> 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...

I suspect it is from raid5_end_write_request.  It looks like we don't print
any message when the re-write fails.  Only if the read after the rewrite
fails.

> but also in this case it doesn't look like a rewrite error...
> 

... so I suspect it is a rewrite error.  Unless I missed something.  What
message did you expect to see in the case of a re-write 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...

Correct.


> 
> > The following patch should address this issue for you.
> > It is*not*  a general-purpose fix, but a specific fix
> [CUT]

NeilBrown