Armor versus archery

[Darth_Roach]

Many of Rome's enemies made great use of bows, namely the Sarmatians, Huns, Parthians and Persians. I wonder how effective was it against the heavily armored Roman soldiers? Any specific types of archery tactics being more effective than others, perhaps? For example the arrow guides the Persians employed to shoot short, heavy bolts from their bows, or nomadic horse archers getting in close before loosing the shot. To sum up - how did archery fare against armor overall?