This looks like beautiful work for your first build, you are a real artist. I have several of this type of structure I built on my homestead used for out buildings. We get lots and lots of heavy snow load and I don't want to clime up and shovel of my roofs so I try to build strong and this means lots of braces. And not to short. As a general rule I like to brace every post on the outer walls and all structural interior posts. Looking at your picture it looks like your second story posts are separate from the first if this is true I would agree with B Redhawk I would brace them. Have the braces go from each corner post down to the first floor beam and joist logs, a mirror image of what's below. Filing the walls in will defiantly stiffen the place up, my woodshed probably wold fall over if I did not keep lots of wood neatly stacked between posts.