Re: Suggestion needed for fixing RAID6

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

----- Original Message ----- 
From: "MRK" <mrk@shiftmail.org>
To: "Janos Haar" <janos.haar@netcenter.hu>
Cc: <linux-raid@vger.kernel.org>
Sent: Saturday, May 01, 2010 7:17 PM
Subject: Re: Suggestion needed for fixing RAID6


> On 05/01/2010 11:37 AM, Janos Haar wrote:
>> Whoever, the sync_min option generally solves my problem, becasue i can 
>> build up the missing disk from the 90% wich is good enough for me. :-)
>
> Are you sure? How do you do that?
> Resyncing a specific part is easy, replicating to a spare a specific part 
> is not. If the disk you want to replace was 100% made of parity data that 
> would be easy, you do that with a resync after replacing the disk, maybe 
> multiple resyncs region by region, but in your case it is not made of only 
> parity data. Only raid3 and 4 separate parity data from actual data, raid6 
> instead finely interleaves them.
> If you are thinking about replacing a disk with a new one (full of zeroes) 
> and then resyncing manually region by region, you will destroy your data. 
> Because in those chunks where the new disk acts as "actual data" the 
> parity will be recomputed based on your newly introduced zeroes, and it 
> will overwrite the parity data you had on the good disks, making recovery 
> impossible from that point on.
> You really need to do the replication to a spare as a single step, from 
> the beginning to the end. You cannot use sync_min and sync_max for that 
> purpose.

You are right again, or at least close. :-)
I have the missing sdd4 wich is 98% correctly rebuilt allready.
But you are right, because the sync_min option not works for rebuilding 
disks, only for resyncing. (it is too smart to do the trick for me)

> I think... unless bitmaps really do some magic in this, flagging the newly 
> introduced disk as more recent than parity data... but do they really do 
> this? people correct me if I'm wrong.

Bitmap manipulation should work.
I think i know how to do that, but the data is more important than try it on 
my own.
I want to wait until somebody support this.
... or somebody have another good idea?

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?

Thanks again
Janos

> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html