You may also want to check for the existence of a
cron job which regularly runs a RAID check on the mirrors. This can look a lot like a resync while it's happening.
On CentOS-type systems, it's done by
/etc/cron.weekly/99-raid-check; I don't know what that'd be on a Debian system, though.
Edit: That's a weekly cron job that runs a RAID check, which causes the discs to perform something very like a RAID resync. This isn't the same thing as just checking to see if RAID has failed; the substantive line is
echo "check" > /sys/block/$dev/md/sync_action. If you're saying that you constantly find your RAID arrays resyncing, this may be what's biting you. If you're saying that they constantly report unrecovering degraded, this isn't it.
If you think this might be it, you'll have to look at wherever Debian keeps its weekly/monthly cron jobs.
Edit 2: this file in
/sys isn't a real file, it's a kernel artefact. You have to find out which
cron job is writing
check to that file, and stop it. I'm sorry, but I've little experience with debian, and don't know where it keeps its system cron files. But if you poke around, you should be able to find the local equivalent of my
/etc/cron.weekly/99-raid-check, and edit it (or a resource file it depends on) so it doesn't do that, or just delete it.
Edit 3: you might try
echo idle > /sys/block/md0/md/sync_action
to stop an in-progress sync check. But it's been a while since I had to disable one mid-check, so I can't swear to that.