I wrote a bit ago about how our servers seem to have surprisingly low power consumption , where I looked at IPMI based power consumption information to look at their current, typical power usage and found that several of them were sitting at lower power usage than my desktops. I'm still interested in typical or average power usage for reasons beyond the scope of this entry, but now I'm also interested in maximum power usage that we've observed.

One reason to be interested in maximum power usage is that servers are often given relatively high capacity power supplies. A 600 watt power supply is on the small side for even a 1U server, and my impression is that 800 watt and even 1200 watt PSUs are reasonably normal in basic servers. On the one hand, the vendors building servers don't know what they're going to be used for, so they're likely to be conservative. On the other hand, that's a lot of spare power for a system that is typically using, say, 24 watts. A PSU that is that far under its rating may not be all that efficient (although the one 600 watt server PSU I looked at had a 'platinum' rating).

Obviously a server's PSU has to support not just its typical power draw but also its maximum power draw; if you idle at 107 watts but reach 500 watts or more at full load ( as one of our servers does ), then you need that big PSU. But if the maximum observed power draw is substantially lower, the PSU looks more like overkill.

Because we only collect IPMI sensor information every minute, I can't be confident that we've captured the absolute highest peak usage for any of our servers. If a server has a high power draw for only fifteen or thirty seconds or so, it would have to happen at just the right time for us to capture that in a sensor reading. But if the power draw lasts for more than a minute, it becomes increasingly likely that we'll capture it.

With that said, the regular 1U servers I have reliable IPMI power usage from top out at around 155 watts for a few servers (over the past year), and our NFS fileservers , which are loaded with SSDs, are under 100 watts. These machines don't seem to be putting much stress on their PSUs, to say the least (I think they all have 600 watt PSUs or better, ie bigger). I don't know what to make of this, or if it matters much from an efficiency point of view. Some extra percent of 25 watts is not necessarily a large number, and I don't know if reducing the PSU from 600 watts to, say, 400 watts would improve things much.

(Even if it did, I suspect that there's not enough of a market for it to make it an economical option for basic server vendors. I have a feeling that most people's servers are more loaded and more power-consuming than ours are.)