Re: Find mismatch in data blocks during raid6 repair

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

Hi Robert,

On Thu, Jun 21, 2012 at 04:58:33PM +0200, Robert Buchholz wrote:
> Hello John,
> 
> On Thursday, June 21, 2012 01:38:38 PM John Robinson wrote:
> > On 20/06/2012 18:41, Robert Buchholz wrote:
> > [...]
> > 
> > > When repairing a full raid6 with no missing drives
> > > (raid-devices=n+2), a single inconsistent data block could be
> > > detected
> > 
> > Yes, it could. See Neil Brown's blog post as to the many reasons why
> > this isn't implemented: http://neil.brown.name/blog/20100211050355
> 
> Thank you for the pointer, I did not find the article before.
> I agree with Neil's premise, this should not be run on a mounted raid as 
> changing data blocks can be a problem, and it should not be the default 
> for resync. However, as both Neil and commenters point out, this is a 
> valuable (offline) repair option.
> 
> Do you know whether the "smart" algorithm or the API necessary to 
> construct a user space program are on the agenda?

a tool is already available, albeit not too much
advertised (my fault, basically).

It is "raid6check" and it is included in "mdadm", at
least in the source repository.

It does *not* repair, but it will tell you which
disk (if possible) and which stripe is incorrect
from an non-degraded (of course) RAID-6.

Any improvement to the code is very very welcome.

Unfortunately I did not have time to follow up the
thing, but a "repair" option might be interesting.

bye,

pg

> 
> Cheers
> 
> Robert



-- 

piergiorgio