> ah, my colleague finally clarified, a resync, not a rebuild, was noticed by
> nagios, glad of that, and i know i'd see it as it happens in /proc/mdstat,
> but surprised to see nothing in /var/log/messages, it looks like
> centos6/mdadm don't log such things?

It should be logged in messages. Resync is enough to worry about hardware
issues and really just about hardware issues. I had noticed a "correction"
down one a RAID6 once. A drive had dropped a sector (remapped it) and that
certainly triggered it. A full-blown re-sync I can only think of being worse:

I am paranoid enough to trigger resyncs by hand as I had indicated before.
But, apparently, packaged system setups do it too:

You can get emails of events with this:
mdadm --monitor /dev/md0 -m myusername at mailserver
You can put this in /etc/rc.d/rc.local or similar place, or just run it
manually with 'at now' from the command line as root.

I did a lot of hacking to test the capabilities of Linux's md RAID. The most
hard-core hack was making 7 sparse files, attaching each of them to a loopback
block device (/dev/loopN), making a RAID5 out of it, and filling it with a
single file that was a pattern of data. Then, I wrote a little C code to go
and modify specific regions of one file; altering the data on the device.
I expected read/write operations on the RAID "array" of files to trigger errors
and rebuild events. It did not. Does not inspire confidence... does it? When
I find some free time I will do some more testing.