QuTS still spreading data to hidden volumes?

[Thisisnotmyname]

I have an HDD pool that I have not yet defined any datasets within but I'm noticing that there's still regular disk activity to/from them. I know QTS used to have a small volume on each array that was used for system files but with QuTS I thought (perhaps incorrectly) that the first pool defined is solely the system volume and all system data is written there alone. Is that not the case? Is QuTS also allocating part of each pool for its own use?
Thank you!