> There just isn't much of a perf penalty. It uses more CPU, yes

Not in my (although small) experience with ZFS vs. databases; the bottleneck is the disk (although it's correct that ZFS is quite CPU-intensive).

Databases have their logging strategies, and ZFS has its own too. So the expense of a write operation is (in principle) doubled.

AFAIK, the typical strategy to mitigate the ZFS overhead is to use a log on a separate, fast storage ("SLOG"). On the db side, in MySQL, it's possible to disable the doublewrite buffer (which is another form of overhead, which, in theory, ZFS should avoid), but nobody really knows if that is 100% reliable or not.