Thank you for sharing that. But it doesn't answer what I am asking. Is there a general sweet spot on a reasonable size to use? I've read cyber's post stating that 120GB should be the max used for 64GB. I've read others saying to use a 4:1 ratio for L2Arc:Arc and now you are posting something that says 5:1. What is the safer bet here? 2:1 or 4:1 ? Or does it not really matter?
Yes it matters. The link I posted comes from the 9.10 documentation of freenas.org
Note the statement "rule of thumb". The sweet spot you ask about is not a concrete number regarding size
due to the many volume configurations and the type/size of files you are serving up with them.
If cyberjock is recommending a max L2ARC of 120GB being max used with 64GB of RAM, you can be sure
that he has tested this, however I don't know under what conditions this was done.
I agree with you about the wide variances, that said, my WAG is that starting with a smaller L2ARC and
some testing would be a good plan. If you find the size lacking after testing, stripe in another.
There's just to many variables (IMHO) to make a sure fire answer to your question.