I worked on the design for an aircraft hanger which included sail shades at the entrance. At that location, wind was a big concern so we talked to some reps for sail shade products. From that research, I'd always recommend sails with four points of attachment in a hyperbolic arrangement (high, low, high, low). 3 points, or 4 points set flat will allow the sail to move up and down in strong winds. They're only resistance to this movement is by being strung really tight. If they get cycling up and down, they can exert quite a lot of force on the anchors.
Some shade sails are made for year round use while some are seasonal. It appears some may only be intended for day use, similar to an umbrella, never intended to see foul weather.
I agree with Nightmare's warnings about anchoring. The anchoring is certainly not trivial. A fascia board is not designed to take that load. From permanent installations I've seen, 6 or 8" diameter steel poles set in concrete foundations seem common for mounting. A smaller size could probably be used if stabilized with guy wires.
@ Isaac-1, I thought I was crazy having a pool and a sailboat (never planned on it) but glad to see I'm not the only one. Interesting idea for the use of old sails. I've always thought that our sailing sails needed to be protected from UV (hence sailbags and sail covers), so I wonder how long they'd last in constant exposure.