Re: the true behavior of mdadm's raid-1 with regard to vertical parity and silent error detection/scrubbing- confirmation or feature request

[Bill Davidsen <davidsen@tmr.com>]

Brett L. Trotter wrote:
> I've googled endlessly about the internal nature of a md Raid-1.
>
> Over the years, I've found several single bit flips on traditional
> platter disks on files that were previously on a linux raid-1 that had
> split and while doing a verification of the two copies. This seems to
> imply that what I'm looking for doesn't exist- and that is simply a
> vertical parity within each disk at the md level- even a single crc32
> every once in a while so that if a bit flips on drive 1 of a mirror,
> drive 2's copy replaces it instead of 1's bit flipped copy replacing
> drive 2's good copy. From what I can gather, it seems to be a 50/50 shot
> whether your good copy gets mangled in the event of a silent bit flippage.
>
> So- is there any built in parity that helps mdadm decide which copy to
> use when the copies disagree on a raid 1 mirror during a resync?
>
> If not- is there a reason why not beyond the extra space overhead and
> read compute write overhead?
>
> This issue interests me more as I look into SSDs and having flash blocks
> wear out.
>
>
> I'd choose a higher raid level if i could, but this is only a very small
> atom 330 box with only mildly important data. I think I'm ultimately
> looking for something like ZFS has, but ZFS under RHEL/CentOS will
> probably never happen in any meaningful production worthy way due
> licensing and the ultimate demise of sun and tainting of things that is
> Oracle.
>
> I'd love any information anyone has on the subject.
>   

I don't think you are going to love this, as far as I can tell there is 
no better recovery done for higher level raid, either, if the failure is 
silent rather than a drive failing. When a 'check' is run and error 
found, Neil seems to believe that it is not worth the overhead of 
identifying the most likely wrong data, so it is simply rewritten to 
make the mismatch go away, rather than to make an attempt to identify 
the most likely correct data for the most likely bad sector and to fix that.

On N>2 copy raid-1, no check is made (unless the change is very recent) 
to see if N-1 copies agree on a value, and with raid-6 the obvious check 
to find the most likely to be wrong data isn't done. This has been 
discussed to death, I don't see any changes coming.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein