And in CORE, persistent L2ARC has to be enabled via
Go to System > Tunables and click ADD . For the Variable , enter vfs.zfs.l2arc.rebuild_enabled . Set the Value to 1 and the Type to sysctl . We recommend noting in the Description that this is the persistent L2ARC activation. Make sure Enabled is selected and click SUBMIT.
as described here. Enabling Metadata-only for a L2ARC is the same for both SCALE and CORE (thank you, @HoneyBadger!).
@jgreco also published some tuning values that worked well for him to get the L2ARC hot and performant. I made some comparisons re: before / after L2ARC as well as before / after sVDEV. Please note that the sVDEV performance may have improved even further since then as I’ve re-organized my NAS as described here. I haven’t had a chance to re-test since then.