In the age of power conciseness and trying to reduce the power footprint in the datacenter, an interesting trend has started to emerge. We've heard of more and more people removing the 2nd power supply from the Dell PowerEdge servers to reduce power consumption. It seems that many people think they cut the power consumption by half since they have removed half the power supplies.

This is an interesting theory to explore, and since we have some of the older servers and power monitoring equipment, we can easily measure the savings. So let's see exactly what the power differences are across two generations of servers from removing the redundant power supply.

Lab Configuration

Dell PowerEdge 1850	Dell PowerEdge 2850	Dell PowerEdge 2950
2 x Xeon 3.6GHz 4 x 2GB DIMMs PERC 4 RAID Controller 2 x 73GB SCSI HDD 2 x 550 Watt PS	2 x Xeon 3.6GHz 4 x 2GB DIMMs Qlogic Fibre HBA PERC 4 RAID Controller 2 x 36GB SCSI HDD 2 x 700 Watt PS	2 x X5460 3.16GHz Xeons 8 x 4GB DIMMs QLA2432 HBA PERC 5E RAID Controller 4 x 132GB SAS HDD 2 x 750 Watt PS

EXTECH 380803 Power Analyzer

Test Methodology

A CPU load generator was used to produce 50% and 100% CPU loads on each system. Power was measured on each system at idle (near 0% CPU utilization), 50%, and 100%. 60 samples of each measurement were taken and averaged for each result. Each test was run with both power supplies in the server, and then again with a power supply removed.

The results were then compared to get the percentage in wattage savings between running the same test with a single power supply and with two power supplies.

Test Results

Dell PE 1850	Single PS Watts	Dual PS Watts	% Difference
Idle CPU	208	235	11.49%
50% CPU	336	359	6.41%
100% CPU	354	374	5.35%

Dell PE 2850	Single PS Watts	Dual PS Watts	% Difference
Idle CPU	230	249	7.63%
50% CPU	365	380	3.95%
100% CPU	385	400	3.75%

Dell PE 2950	Single PS Watts	Dual PS Watts	% Difference
Idle CPU	288	299	3.68%
50% CPU	394	408	3.43%
100% CPU	438	446	1.79%

Conclusions

Not 50% savings, but some savings.

You can see that in our configurations, we are no where near the maximum power ratings stamped on the power supplies. Power supplies "supply" only the power that the components in the server are demanding. With two power supplies in the system that demand load is spread across the supplies in an equal manner.

Two major conclusions from looking at the results. First, with older servers there is a bigger savings.

Second, as the load on the server goes up, the savings goes down.

Are the savings worth the loss of a redundant power supply ? I don't know, you have to tell me. Please weigh in your arguments below by starting a thread.