ZFS doing background writes?

[Dirk E]

I'm curious now; does ZFS do any I/O on the background?

Because i'm getting periodic writes from ZFS to my disks, beginning right after i import the pool.

Even going to single user mode does not help; zpool iostat -v 1 still shows the writes with static 5 second interval.

What would cause this behaviour? I encountered it with both 10.1-RELEASE and a very recent HEAD.