# Presentation: 203844

## Building AComputer Centre HEPiXLarge Cluster SIGOctober 21st 2002Tony.Cass@CERN.ch

## Why do you need a Computer Centre?

- Because you have some combination of CPU servers, Disk Storage and Tape Storage.
- All of this equipment
- consumes electrical power
- which may need to be protected against failure
- turns electrical power to heat at 100% efficiency,
- needs to be put somewhere, and
- needs to be protected against fire.
- Your Computer Centre or Computer Room must
- have an adequate power supply
- have adequate cooling,
- be large enough, and
- have smoke detection and, possibly, fire suppression.

## Electrical Power

## How much power do you need?

**Tape drives and robotics are low power**. O(100W) per device and you don’t have many of them.
**Disks are also low power**. O(10W) per device.
- Main concern is inrush current as drive spins up. Don’t start them all at once.
**CPUs are the problem**. **Lots of them** and **power efficiency is not improving** as raw performance improves.
- A study of processor specifications suggests constant power consumption of **1W/SpecInt95**.
**Power factor** of the power supply is **important**. Off the shelf supplies have power factor of **~0.7**.
- Power supplies with power factors **better than 0.8** are available, but these are generally **more expensive**.

## PC power supplies only draw current when the voltage exceeds a certain threshold:

Result? Lots of harmonics, especially 3rd. These lead to high currents in the neutral conductor.
Electrical distribution can need neutral conductor with twice the cross section of the phase conductors.
All the way from room distribution, through PDUs to the transformers.
c.f. no neutral needed for ideal 3-phase. US building code used to allow half cross section neutral conductors. No longer!
Potential fire risk.

- Result? Lots of **harmonics**, especially **3****rd**. These lead to **high currents in the neutral **conductor.
- Electrical distribution can need **neutral conductor** with **twice the cross section** of the phase conductors.
**All the way** from room distribution, through PDUs to the transformers.
- c.f. no neutral needed for ideal 3-phase. US building code used to allow half cross section neutral conductors. No longer!
**Potential fire risk**.

- Switched Mode Power Supplies

## Power Protection—High Level

- Usual question: “What happens if your primary supply fails?”
- Better question: “**How long does it take you to perform a controlled shutdown?**”
- My guess? **Over half an hour**. Probably **2 hours**.
**High power, long autonomy** battery backed (static) **UPS systems are expensive**. **Need diesel** backed (rotary) systems **for decent autonomy**. These are also expensive!
- If you can, **install a 5 minute static UPS to smooth power and cover microcuts**.
**expect to lose physics computing due to power cuts**.
**consider a 2 hour static UPS for critical systems** (networks, databases, ...) if you have no diesels.

## Power Protection—Local

- Many “multi sockets” come with **differential** (or earth leakage) **circuit breakers**.
- These trip on small difference (usually 30mA) between the phase and neutral currents.
- This is fine for general home use, but **bad for connecting many PCs**.
- Power supply design gives natural difference of 3-5mA. If you have lots of PCs behind one differential breaker this may trip if there are small disturbances.
- This is **our theory**. We will try omitting these breakers for new installations.
- But need to provide dedicated sockets for, e.g. consoles.
**Ask me for an update next year**...

## HVAC

## HVAC Options

**Water cooling**
**Efficient**, **but** **no pipes in equipment today**. Does any vendor want to be the first to add these?
- IBM Ice Cube would have internal water cooling, but even here, closed system is mentioned as option.
- HP talk of using “ink jet” heads to spray water on cpu. This cools the chip faster, but the heat still ends up in the machine room.
- Can put water cooled pipes in racks (common practice in online areas for experiments) but rack layout has to be fixed.
**Air cooling**
- Air conditioning units are connected to the chilled water supply and circulate chilled air in the machine room.
**Flexible**, but...

## Air Cooling

- ... Heat capacity of air is limited. Extracting **lots of heat** means **lots of air changes** per hour.
- Acceptable level depends on room usage and occupancy.
**above 60-70 AC/h** the environment becomes **uncomfortable** for general work.
**Good! Make SysAdmins work from their offices!**
**120AC/h acceptable for technical areas**
- If possible, **avoid closed systems**. Injection of external fresh air allows “free cooling” in winter.
- Cold air injection usually under false floor
- need to ensure air flow through equipment racks.
- Roof level injection possible, but requires high ceiling.

## Air Cooling Limits at CERN

|  | Clearance |  |
| --- | --- | --- |
| Air Flow | 2.5-3m | >6m |
| Comfortable | 600W/m2 | 1200W/m2 |
| High | 1200W/m2 | 1800W/m2 |

- For guidance only: your mileage may vary...
- Don’t forget to account for overheads (e.g. solar heating)! These depend on location, but we have 3-400W/m2 for the main machine room in summer.

## CERN Planning

## Space

## Space

**Box size may not be the determining factor**!
- 40 1U boxes in 19” rack: 4kW in 1m2 (including clearance).
- So, good if you can **mix CPU and disk servers**.
- Better still if you have tape robots!
- Otherwise, space is relatively easy. Just **remember** to add enough **clearance**.

## Fire Precautions

## Smoke Detection

**Laser based** smoke detection systems are **extremely sensitive**.
- Use these to **raise early alert**, but
**need less sensitive detectors as “second knock**” to trigger any fire suppression or prevention.
**Localisation is a problem**.
- The combination of sensitive detectors and rapid air flow means all detectors will trigger in time—usually sooner, not later.
**Locality of smoke detection competes against efficient cooling.**
- You can divide a big room into smaller ones, but it will be harder to cool.
- The same is true for closed racks. And, anyway, smoke will escape into the room eventually.

## Fire Suppression or Fire Prevention?

**Fire Suppression if you want to keep services running** at all costs (e.g. banks, phone companies, ISPs).
- Usual system is to **flood machine room with an inert gas** (Nitrogen/Argon mixture now that Halons are banned).
- Gas is expensive and bottles are bulky.
- Machine room needs to be hermetic!
- Some modern water based systems can be used on running equipment and are good for scrubbing smoke particles from the air.
**Fire Prevention** is the alternative **if you want to avoid fire at all costs**.
**Cut electrical power as soon as smoke is detected**.
- Risk of progression to fire cut by 20x-100x for smouldering PCBs
**Fire Prevention chosen at CERN** for our new machine room in the former tape vault. Ask our opinions next year...
- We’re still not sure what to do for our main machine room because of the localisation problem.