Re: what does md do if it finds an inconsistency?

On Sunday May 6, gmccullagh@gmail.com wrote:

On Sun, 06 May 2007, martin f krafft wrote:

quoted

Maybe the ideal way would be to have mdadm --monitor send an email on
mismatch_count>0 or a cronjob that regularly sends reminders, until the
admin logs in and runs e.g. /usr/share/mdadm/repairarray.

You could certainly do that.  If you configure mdadm to run a program
for each 'monitor' event, you can detect the mismatch count from
argv[3] when argv[1] ==  RebuildFinished.

Though I suspect many people would be happy with running the 'repair'
every month rather than just a 'check'.  Maybe that should be a config
option.

quoted

Also, if a mismatch is found on a RAID1, how does md decide which copy is
mismatched and which is correct? What about RAID 5/6/10?

I think it just picks one at random.  After all, how could you reliably
know which is right in a raid1 array?  With raid5, I understand it just
updates the parity.

I prefer to say "arbitrary" rather than "random".
I think the current implementation uses the first readable device as
the 'correct' one.
Otherwise, this is correct.

I had an idea to write an interactive userspace program which ran through
each block on each disk device to figure out which ones didn't match up and
then figure out whether it's within allocated filesystem space and if so,
which file or filesystem data was affected.  This would hopefully enable a
user to figure out which block is wrong and correct things.

That would be awfully difficult as doing a reverse mapping (block ->
file) is no-trivial in almost any filesystem, and you would want to
(ultimately) do it for every filesystem...

Might be educational though :-)

NeilBrown