Bug#658701: mdadm: should send email if mismatches are reported by a check

[NeilBrown]

On Sat, 26 May 2012 18:39:08 +0400 Michael Tokarev <mjt at tls.msk.ru> wrote:

> Neil, can you comment on the change to Monitor offered
> in the mentioned bugreport please?
> 
> On 12.04.2012 23:28, Michael Tokarev wrote:
> > Neil, re http://bugs.debian.org/658701 , how do you think,
> > is it okay if mdadm --monitor will send email in case check
> > found mismatches, the same way it sends email about other
> > more critical errors?
> > 
> > I think Russell has a good point here, but there's one more
> > source of mismatches we have in kernel - some "sporadic"
> > mismatches in raid1 and raid10, especially when these are
> > used as swap space...
> > 
> > In Debian we've several bugreports already requesting more
> > attention to mismatch_cnt, see:
> > 
> >  http://bugs.debian.org/658701 (this one)
> >  http://bugs.debian.org/599821
> >  http://bugs.debian.org/588516
> > 
> > Thank you!
> > 
> > /mjt

Sorry for not replying the first time :-(

I do not agree with the suggested change to mdadm.
A non-zero mismatch count may not be a problem.
It could be due to swap writing to a RAID1/RAID10.
It could also be due to a RAID1/RAID10/RAID6 having been
created with --assume-clean.  This is perfectly safe thing
to do but results in a non-zero mismatch_cnt.

mdadm --monitor will run a program on every event.  If someone
wants more events reported than currently are reported, they are
free to write a script to do whatever they like.

If md finds unreadable blocks and fixes them, then that certainly
might be interesting.  However that is interesting much more broadly than
just for md, and I believe 'smart' makes that information available.  So
having it reported from SMART would be more sensible.

In brief: mismatch_cnt maybe useful to someone who understands what is means
and is investigating some issues, but it is not something that should be
automatically reported to a casual sysadmin.

NeilBrown
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