I think that is well known amongst storage experts, though maybe not everyone who might be interested in using ZFS for storage in a professional or personal application. What I’m curious about is how ZFS’s full-disk performance (what is the best term for this?) compares to btrfs, WAFL, and so on. Is ZFS abnormally sensitive to this condition, or is it a normal property?

In any case it doesn’t stick out to me as a problem that needs to be fixed. You can’t fill a propane tank to 100% either.

ZFS has gotten significantly better at 80%, but 90% is painful enough that I almost wish it would reserve 10% a bit more explicitly (maybe like the old Unix systems that would prevent non-root users from using the last 5% of the root partition).

All my arrays send me nightly e-mails at 80% so I'm aware of when I hit there, but on a desktop system that's typically not the case.