As I understand it, computer rooms in the days of mainframes and minicomputers, commonly had raised floors, so that the space under the floor could be used for power cables and cool air.
Mainframes and their peripherals, yes, minis to a way lesser extend. After all, one of their main advantages was to not being placed in a computing center.
I'm not clear on why this was considered better than running these things above the floor,
Two main reasons:
In many ways the cabling part was in the early days even more important than the cooling aspect. Interface cables weren't tiny, less than 5mm tick ones, but several cm in diameter, quite heave and resisting bending even more. Last but not least, as shorter a cable was, as faster an interface could operate. Putting cables underneath solves all of this:
- no weight restriction (like on overhead cradles)
- much room for wide curvature, so no tight bending
- straight connections
And no, the same could not be archived by putting devices side by side there would be simply no room to lay the cabling, nor could all devices of a machine be put in a single line. Having 10+ disk drives was not unusual - each the size of a fridge plus two controllers, each two racks plus the CPU plus operator console (yes, they were connected with interface cables, not serial lines) plus a tape controller (at least a rack) and a rack per tape, plus a printer or Two, each the size of a Smart car. So no, no way to put that in a row and have all cables go above ground.
For the cooling part, it simply follows physics of hot air going naturally upward. Devices were constructed in a way to acknowledge this, taking air in from below and spilling it atop.
This is still the same for today's machinery, although it did change to some degree due different construction.
Apparently, raised floors are going away; modern server rooms tend not to use them.
This depends still a lot on the value of $SERVER_ROOM :)
I know many data rooms not only still using raised floor, but as well new ones build with raised floor.
If they were considered advantageous before, why no longer? What's the difference between a mainframe computer room and a modern data center that makes a raised floor worth its cost in the former case but not the latter?
Beside less careful planning and use of non dedicated rooms?
It's different structure of the devices to be installed. For mainframes, each device was a rack of it's own, if not multiple. Each needed a power cable and one or more interface cable (usually in pairs). All to wire up a single computer. Nowadays the whole computer is a 1..3 HE slab pushed into a standard rack with only a single, thin (~1 mm2) power cable and one or two very thin LAN cables. Both are light wight and easy to bend cables, so many of the classic restrictions no longer apply.
Both may be routed overhead as well, although, it's quite convenient to have all data go overhead and all power below, using again a raised floor. This may as well be enforced by local code asking for strict separation of mains power from any other cabling.
Next, cooling intake is different. Rack mounted servers have a different airflow. Air is taken in at the front and released from the back. It's no longer as important to have it coming straight from the bottom and at comparable high pressure.
Last but not least, they need way less cooling. While density has improved, dissipated power per rack hasn't much. It may even have declined, at least when comparing with mainframe CPU racks. This is especially true with setups using external power supplies. That is having one large PS per rack (or multiple racks) converting main line AC to low voltage DC. Using these results in a more effective, less heat producing setup. At the main converter as well as with every connected server.
The same goes for offloading disks into dedicated disk racks. HDD are a prime source for heat. By having (again) dedicated storage racks, their cooling needs can be tailored in better ways than intermixed with CPU servers.
While raised floor has lost its appeal for cabling, it's still a prime estate for cooling and power distribution. Just, now maybe only 25-30 cm deep (<1ft), compared to 50-70 (>=2ft) for 1970s raised floor.
So, long story short, the kind and structure of installed devices have changed and so has computing center floor design.
There is a point in Stephens Answer (who in general may be way more up to date than an old fart as me) I would like to tackle:
But raised floors don’t cope well with constant change: the area beneath the floors is hard to access, especially parts under cabinets, and routing cables etc. can quickly become a nightmare.
I beg to disagree here. Access is extreme easy, Pulling one (or more) tiles to route cables is way less work than fetching a ladder and walking along the overhead bin, constantly repositioning the ladder - or pushing a wheeled ladder.
(Note that overhead wiring may still be preferable, see above).
Raised floors also limit rack density [...] because you need to be able to actually lift the tiles.
This is what data center planning is all about. pick the right floor and place the racks accordingly, so the walkway is always full tiles that can be removed without moving racks.
But I give that this might be a lost - or better unknown - art to many people nowadays. The floor is often planned independent from what is to be installed. "Back in the olde days" floor planing was a dedicated stage before setting up a machine, even before buying the floor. I would have thought it be even less of an issue today with next to all racks following the same 19" form factor. Unlike in the 70s where disks had a different footprint than tapes, both different from their controllers which again differed from CPUs and other devices.
Another point about the cited article:
Another example is you oversize your air-conditioning system, and there’s excessive airflow going underneath the raised floor. The airflow is moving horizontally faster than it should [and] actually causes a Venturi effect […]
This again is a result of bad computing center design - like having air coming in from only one inlet at one side - assuming a floor would automatically distribute even. That didn't hold in the 1970s nor does it today. Anything past a few square meters needs multiple inlets. In not due eternal routing than by laying ventilation shafts as well within the raised floor, equalizing air flow, so there are no 'crosswinds', but an (almost) constant pressure resulting in an even upward flow at all locations
This is especially true for modern installations that try to be more 'flexible' by not adding cut outs under each rack, but layering the whole room with tiles that allow airflow.
That might be in fact a detail missing for many when thinking about raised floor and cooling. It wasn't about letting air out on all tiles all across the room, but have dedicated cut outs only below each rack/device. That way the raised floor did work like a system of tubes going directly to each device, cooling the devices, not the room at whole.