Many people (like me) use "halon" when they are talking about general inert gas based fire suppression as there's not really a good short word for them otherwise.
So fire suppression and data centers don't mix too well
https://www.hack.org/~mc/texts/vaxen-joke.txt
Realizing that I'm spoiling some of it... but we're here, so some of it is inherently spoiled...
The fire alarm klaxon went off and the siren warning
of imminent halon gas release was screaming. We
started to panic but the data center manager shouted
over the din, "Don't worry, the halon system failed
its acceptance test last week. It's disabled and
nothing will happen."
He was half right, the primary halon system indeed
failed to discharge. But the secondary halon system
observed that the primary had conked and instantly
did its duty, which was to deal with Dire Disasters.
It had twice the capacity and six times the discharge
rate.But yes, I've seen low-O2 installations also. These have other issues though, like no protection when humans need to be in there for maintenance. And the need to be very very careful when reducing it again, making sure everyone is out. It happened to my colleague once that this went wrong and while it's not low enough that you'll die, he wasn't exactly feeling great for the next few hours.
But it's only used in military and aviation these days anyway.
You just have to reinforce the ceiling panels and the windows so they don't get blown out, but it's not really an issue otherwise. It was all certified and regularly checked. Water is indeed a bit counterproductive as it's not really much better than fire for the equipment in there.
PS: Halon itself is of course gone but it's other less harmful gases these days.
What also happens is low-oxygen environments (they only raise the O2 when maintenance is being done).