Not a data centre expert, but don't they quite famously pump CO2 into the server rooms automatically if the fire alarms go off, even if there are people in the room?
Typically server rooms are currently Fluoro-K / Novec123 if they have space constraints on the suppression systems. Those cylinders take up the least space per cubic foot of protected environments. These are less harmful than previous gasses like Halon, but still not good for the environment.
If you're lucky we get given ample room for cylinders and can go with Argon suppression systems, but that can take up more than 4 or 5 times the space alone and adds a lot of cost in engineering manifolds, etc.
CO2 takes up as much space as Argon systems and will kill someone who doesn't leave the hazard area before release so its generally not used in any datacenter project I've done.
Powerplant's at datacenters often use drychem systems instead or foam releasing systems.