Problem/Justification
The dashboard panes can show the pool fill, RAM fill, etc. However, the fill level of the sVDEV is a mystery unless one drops into the CLI or uses the Disks GUI pane to show fill levels.
Impact
Since an overflowing sVDEV could have dramatic impacts on pool responsiveness or small file IOPS, being aware of sVDEV fill (of the metadata and small file partitions) would be helpful. There is little downside, other than the need to parse out the metadata vs. the small file fill.
User Story
I love my sVDEV but I also recognize that fusion pools make the diagnosis of a throughput problem more difficult because there are more things to juggle. A top-line review of sVDEV partition fills would give early warning to admins that pool performance may start cratering shortly.