I've upgraded a box to 11.4, and it's constantly writing, in a cyclic pattern about 1-2Mb/s to disk. What's it doing? It didn't do this on 11.3, and poking about with `iotop` and `rwsnoop` only tells me that the `zpool-rpool` and `zpool-space` (space is my data zpool) are doing the writing. I don't see anything on the SSD cache and log devices when this is happening, only the spinning disks, and the two halves of the mirror have identical write patterns. I'm pulling the data out of kstats.
The chart above is typical. You can see "real" writes because they hit the SSD as well (blue, at the bottom). The system normally runs about a dozen zones, but shutting them all down, and with the global doing nothing, I still see the pattern.
It's not a huge problem, but it seems odd. I'd be very interested to hear any explanations, or pointers for further investigation.
Rob