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systemd mdadm spamming my syslog

Since a recent update, my /var/log/syslog is getting spammed with huge
numbers of messages of the form

Mar 10 14:02:25 snowball systemd-udevd[18681]: Process '/sbin/mdadm --incremental --export /dev/sda3 --offroot /dev/disk/by-id/ata-ST3000DM001-1ER166_Z501MTQ3-part3 /dev/disk/by-partuuid/ad6f31ee-1866-400c-84f8-2c54da6abd2e /dev/disk/by-path/pci-0000:00:11.0-ata-1-part3 /dev/disk/by-id/wwn-0x5000c50086c86ae8-part3' failed with exit code 1.

When I run the command by hand, I get

root@snowball:~# /sbin/mdadm --incremental --export /dev/sda3 --offroot /dev/disk/by-id/ata-ST3000DM001-1ER166_Z501MTQ3-part3 /dev/disk/by-partuuid/ad6f31ee-1866-400c-84f8-2c54da6abd2e /dev/disk/by-path/pci-0000:00:11.0-ata-1-part3 /dev/disk/by-id/wwn-0x5000c50086c86ae8-part3
mdadm: cannot reopen /dev/sda3: Device or resource busy.

Which at least gives me a small clue, but really not much of one.

I'm not even really clear on whther this is a systemd or mdadm bug.

So, some questions:

1) what is this command trying to do?  I do understand a little about mdadm,
and am running a RAID 1 array on this machine.  But this is using an
option (--offroot) that doesn't even appear in the man page, and I've
got no idea what it's trying to accomplish.

2) how can I make it stop?