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Understanding Power for POWER

Designing for Power

Much of the confusion comes from the way redundant power supplies work and the requirements this puts in place for additional PDUs. As an example, the S824 has four power supplies set up as two redundant pairs. This means that two cords should go to one PDU and two should go to another so if a circuit or PDU fails the server can continue to run. For the most part, two of the power cords may never draw power, however we don’t get to state which ones will. This means that redundant power supplies can hide the problem of accidentally exceeding the 4.8 kVa loading on a 30-amp PDU in normal operation, since we’ll never see one of the PDUs in the pair being used. However, if one PDU in a redundant pair happens to fail—for example, if a PDU cord is unplugged or an input circuit fails—the entire load of all the devices will be forced to the remaining PDU where it’s likely to fail due to the overloading. I have seen this happen several times and the result is never good.

So the first thing you need to determine is whether your circuits are 60 amp or 30 amp. POWER7 and POWER8 servers draw plenty of power so wherever possible it’s recommended you go with 60-amp circuits. If you remain with 30-amp circuits, you’ll need to provide a lot more PDUs. Once you determine the amperage and voltage, then it’s a simple case of planning connections such that you don’t exceed the rules for power usage.

Here’s an example for a system consisting of an S824, a three-node 770 MMD and an HMC and monitor.

Using the maximum use numbers from the Technical Overview Redbooks: Each 770 node is 1.649 kVA and 1,600 watts, the S824 is 2.38 kVA and 2,300 watts and the 7042-CR8 HMC is about 0.55 kVA and 523 watts. The monitor requirements are negligible. So for planning purposes we need:

			kVA			Power Plugs 
3 node 770		4.947			6 (3 pairs)
S824			2.38			4 (2 pair)
HMC			0.55			2 (HMC) + 1 monitor
TOTAL		7.877 kVA

If we’re using 30-amp (derated to 24 amp) PDUs, then we can only support up to 4.8 kVA on a PDU so the example configuration would require four PDUs (two to support the 7.877 kVA primary needs and two for failover). With 60-amp PDUs, we can support up to 9.6kVA so we would only need one pair of PDUs (one PDU for primary and one for failover). It should be noted that the E870 and E880 have higher power requirements with each node potentially drawing 4.2 kVA across four power cords. This means that a pair of 30-amp PDUs can only support one E870 or E880 node and nothing else.

For Your Safety

IBM has taken great care to ensure its support statements adhere to the electrical codes. Even though you can use the energy estimator to determine numbers closer to what you would really use, it’s still important to plan for enough power to support the servers if they were to have all the resources activated or in use. I normally plan for power using the energy estimator as well as the published maximums, and I use the larger of the two numbers. I don’t expect to draw that amount of power, but to be supported and in compliance with electrical codes, I must plan for the potential additional power. Plus I also ensure that the requirements for redundancy are taken into account and that IBM’s rules for maximum servers per PDU are followed. This ensures that the servers are fully supported and that PDU failure doesn’t cause servers to go down due to lack of power or, worse, cause fires. These redundancy requirements along with the regulations mean that you need to plan for what you could use, not just what you’re currently drawing when all is normal. This is why it’s important that, any time a server or I/O drawer or similar is added, a review should be done of the PDU layouts and usage. This is also why it’s important to have a preinstall planning meeting before servers or upgrades are ordered.

Jaqui Lynch is an independent consultant, focusing on enterprise architecture, performance and delivery on Power Systems with AIX and Linux.

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