Re: User space RAID-6 access

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

On Tue, Feb 01, 2011 at 07:52:59AM +1100, NeilBrown wrote:
> On Mon, 31 Jan 2011 21:20:55 +0100 Piergiorgio Sartor
> <piergiorgio.sartor@nexgo.de> wrote:
> 
> > Hi all,
> > 
> > some times ago, I think was Neil, it was mentioned
> > about the possibility to access consistently a RAID-6
> > array from user space, in order to be able to perform
> > some checks, like the notorius "which HDD has wrong data".
> > 
> > Is there any reference or documentation or source code
> > which can be taken as example for such a case?
> > 
> 
> Look in the mdadm source code, particularly at restripe.c
> 
> Also
>    make test_stripe
> 
> make a program the the test suite uses for verify data correctness.
> 
> That should give you enough hints to get you started.


Hi Neil, thanks for the pointer.

I had a look at the code and there is something I did not get.

It seems to me the HDDs composing the array are "simply"
opened with "open".

In case the array is in use, how is avoided a race condition
between the "test_stripe" program and the md device?

I mean, the first could start to read from the HDDs and the
other could write something in the same place, leading to an
inconsistent parity, from the "test_stripe" point of view,
since it missed some update.

Or there is a locking inside md (in "test_stripe" I could
not see and it would also be dangerous)?

Thanks again,

bye,

-- 

piergiorgio