Re: mismatch_cnt again

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

Hi,

> I think just "block ABC is inconsistent" is sufficient.
> user-space can then quiesce that part of the array, read the relevant
> blocks, do any analysis that might be appropriate, and report to the
> admin. 

personally I think user space is good for
this kind of operations.

I think the point here is not if this kind of
recovery should be in kernel space or not, but
to have this kind of recovery.

> That is a very interesting threat scenario - occasional bit flip on
> read between media and memory.  I had a drive like that once.  One
> particular bit in the sector would fairly often return '1' no matter
> what had been written.  I had it in a RAID1 and it quickly made a mess
> of the filesystem.

In my case, a further analisys showed that the "bits"
where always *written* correctly, but the reading
operation was, sometimes, flipping bits.

This was especially nasty, because, without "resync"
the array would have been always fine.
 
> As you say, there is nothing that md can or should do about this
> except report that something odd is happening, which it does, and
> report where it is happening, which it does not.

Well, md specifically may or may not have the infrastructure
to use the RAID-6 parity to correct this sort of issues.
Nevertheless, using the RAID-6 double parity, in user or
kernel space, is really one point for software RAID.

bye,

-- 

piergiorgio