It is an observability issue, even zabbix tracked reserve space and inodes 20 years ago.

Will dedupe,compression,sparse files you simply don’t track utilization by clients view, which is what du does.

The concrete implementation is what matters and what is, as this case demonstrates, is what you should alert on.

Inodes, blocks, extents etc.. are what matters, not the user view of data size.

Even with rrdtool you could set reasonable alerts, but the heuristics of someone exploding a sparse file with a non-sparse copy makes that harder.

Rsync ssh etc… will do that by default.