While building out a new data center, we took a look at what our practice had been for provisioning power for servers and devices within a rack. As pretty much everyone does, we bring two separate 220v circuits into each rack. One circuit terminates on some kind of power distribution unit (PDU) on one side of the back of the rack, the other circuit terminates on a different PDU on the other side of the rack.
When we provision a new rack and install equipment into the rack, obviously we carefully route and label all the power cables and make sure that each power supply on a dual powered server or device is connected to a different PDU. We also make sure that if we have an HA pair of single power supply devices, one device is connected to each PDU. We typically connect the left power supply to the left PDU and the right power supply to the right PDU. In most cases we test the power by turning on all the servers in the rack and intentionally failing each PDU, one at a time. In theory, if either circuit fails, any dual power supply devices will alarm us on a power supply failure, the load will transfer to the other PDU (which will not be overloaded, right?) and your application will not notice. The HA pair of single powered devices will do a failover and failback.
Everything is good, live goes on.
Until a few years go by, a few servers get re-racked, and a few vendor tech's swap a few server parts. Eventually someone will plug something in wrong, and during the next circuit failure you'll have unexpected downtime. (Don't ask me how I know....). The question then is: How do you know that you still have proper power redundancy after the rack has been tweaked around with for a few years?
Red power, Blue power
We bought a couple rolls of colored electrical tape from the local big box home store and wrapped a band around each end of the power cords connected to one PDU in one color and the power cords connected to the other PDU with the other color. (For us, red on the right side, blue on the left.). Now a quick glance at the back of a rack after intrusive maintenance will tell us if we have properly attached dual power supply devices, and more importantly, will tell is that our redundant pairs of single power supply devices are each connected to separate power. Mismatched colors stick out like a sore thumb.
I hate it when I sound like Martha Stewart.