PSU

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Introduction

Choosing a PSU (power supply) for unRAID is not as simple as it first seems. This page will help you understand the basics, and help you understand the reasoning behind the selected PSU's on the suggested purchase list below.

An unRAID server is not like the typical home PC, with rarely more than three hard disks. Your unRAID server may eventually have 10 or more disks installed. Each one of these drives have power needs that change at different times during operation (i.e. first power up, under load, spun down, spin up, idling, etc).

So how does this affect your choice of PSU? Well, how much power your PSU can provide is important, but what is more important is how it handles the peaks, how well it runs during normal loads, and how much power is available from the individual power cables.


Caveat: There are a lot of "rules of thumb" and "typically" statements in this mini guide. Assume nothing when making your final purchase decision, check everything.


Myths and Mistakes

There is no international standard that manufacturers follow to describe the capabilities of their power supplies. This results in manufacturers lying or at least intentionally misleading you with regards to PSU specifications.

The wattage rating of a PSU (e.g. 550 Watts) is the single most misleading figure. You cannot simply add up the power requirements of your equipment, and buy a PSU with a wattage figure that exceeds it, and expect to have a stable system. The following sections will help you with the important details of what really matters in specifying your power supply.


What's Important

So WHAT is important?

Rail Count

A PSU can deliver a certain amount of power overall. However a multi-rail PSU (which 99.9% of them are) can only deliver a portion of that power per rail (or in real world terms per cable). The ATX standard specified that a maximum of 20 Amps can be output on any one rail. The problem with this is systems with high HDD count can easily exceed this figure on boot up.

Luckily some higher-end manufacturers have said "ATX" be damned and produced single-rail PSU's. These supplies can deliver their full load from any one of their cables, meaning that you can connect your drives as you see fit. (Some PSU's claim to be multi-rail, but are actually single-rail.)

If you do nothing else, make sure you buy a single-rail PSU. (But see http://www.jonnyguru.com/forums/showthread.php?t=3990)


For unRAID specifically, we care primarily about the +12V rail(s). When shopping for a PSU, look in the tech specs for a list of numbers like this:

+3.3V@24A, +5V@24A, +12V1@20A, +12V2@20A, -12V@0.5A, +5VSB@2.5A

As far as unRAID is concerned, we can ignore all of them except for the +12V rails. The above example has two +12V rails (note +12V1 and +12V2). So ignoring the rest, we have:

+12V1@20A, +12V2@20A

The A stands for Amps, so in this example the PSU has 20 amps on each of its two +12V rails. A modern 5400 RPM 'Green' drive uses about 2 amps, whereas a faster 7200 RPM drive can use anywhere from 3 amps to 7 amps, depending on the make and model. Older drives generally use more power. So a 20 amp rail can support about 10 green drives or 4-6 7200 RPM drives.

I wish that were it, but there's more. Since this particular PSU has two rails, it is difficult to know how those rails are distributed across the actual cables that come out of the PSU. Judging by the pictures, it looks like this PSU has one cable for SATA power and one for Molex power. In this case, you might think we can assume that each of these cables gets a single +12V rail and balance the build appropriately. That is almost universally NOT true. What if it had 3 or 4 SATA and Molex cables, as many PSUs do?

If you are savvy you might be able to consult the manufacturer's wiring diagram and perhaps be able to figure it out... if they give the details. Unfortunately, only a few power supply manufacturers provide a description of which connectors are powered by which rails. In every case I've personally investigated, on a multi-rail supply, only one of the 12 volt rails is used for the all the Molex and SATA connectors, and it frequently is also the same rail that powers the 24-pin connector for the motherboard. A multi-rail 500 watt supply may actually have less power available on the one 12-volt rail used for the hard disks than a 300-watt single-rail supply.

See this post: [1] for an example of a high-quality four-rail 550-Watt supply that has far less capacity for disks than you might guess. All the disk and SATA connectors share the same rail as the 24-pin motherboard connector. The disks might have 15 Amps capacity available... and just think of the potential for problems if the peak demand causes the voltage to the motherboard connector to be unstable. I don't think it is safe to use more than 5 or 6 green drives with this supply.

This is why we recommend that you choose a PSU with a single +12V rail. Doing so means that you don't have to worry about balancing loads (since in most cases you cannot balance across multiple rails), you can just plug in everything where it fits and it will work. You do still have to pay attention to that Amps number, however, since it will determine how many drives and peripherals your PSU can support.

Here's an example of a good-quality Corsair 400W PSU with a single +12V rail:

+3.3V@20A, +5V@20A, +12V@30A, -12V@0.8A, +5VSB@2.5A

30 amps on a single rail means that you can support up to 15 green drives or 5-7 7200 RPM drives.

Efficiency

PSU's operate at different efficiencies depending on how much load they are under. Efficiency is essentially how much AC you are pulling from the wall to produce the DC you need for your computer. As a rule of thumb, a PSU is most efficient delivering 50% of its maximum output, e.g. a 600W PSU works best delivering a constant 300W.

It is beneficial that you try and spec your PSU to operate in the efficiency sweet spot. It won't make a huge difference, as efficiency rating on a good PSU only changes +/-5%, but it should demonstrate that buying that 1000W monster PSU could actually cost you more to run.

In reality, this will be hard to do as HDD's pull 2-3 times more power on startup than at any other time. This makes it hard to find a PSU that will cope with maximum startup load, while still operating at 50% rating during normal operation.

To counter this, buy a PSU that is at least rated at 80% efficient. Choosing a PSU that is generally more efficient will save you money in the long run, using less electricity than a 70% efficient PSU (on a 24*7 server this will make a difference over a year).

Noise

Most unRAID users operate their servers in a home, so noise is an important variable. Like the previous efficiency section PSU's typically operate quietly up until about 50% of their maximum output wattage. If in normal operation you are running at 50% or less, your PSU will be quiet. Noise levels above the 50% mark can increase by as much as double every 75 Watts of load.

Heat

Heat and Efficiency go hand in hand. An 80% efficient PSU loses most of that 20% as heat. The harder a PSU has to work proportionate to its maximum capacity the more heat it produces. However up until the 50% of full load mark, you shouldn't be able to detect much of a heat difference.

At a minimum, make sure you buy a PSU with a 120mm fan. Quality manufacturers generally use high end, low noise, mostly ball bearing fans of at least 120mm size. Low end manufacturers will skimp and use smaller, noisy but flashy looking, fans.


Example

The final decision on PSU purchase is up to you, but the forums are full of success stories with a short list of models. To help you understand what calculations you need to conduct, the following is a VERY rough example of a fully loaded unRAID system (note: these are not accurate figures, research and use your own):

  • CPU = 50W
  • Motherboard, memory, on-board GPU = 40W
  • 6 Fans = 1.5W * 6 = 9W
  • 2 Controller cards = 10W * 2 = 20W
  • 17 HDD's = 28W * 17 = 476W
  • Total = 595W

So on startup this system needs to be able to support at least 595 Watts.

However during normal operation, these HDD's only draw 7W each, and normally we may have only 5 drives spun up, and the rest are spun down, which is 1W per drive. So:

  • 5 HDD's spun up at 7W each = 35W
  • 12 HDD's spun down at 1W each = 12W
  • HDD total = 47W
  • System total = 166W

You can see that in normal operation our power requirement for HDD's has dropped by almost 430 Watts, and our system power requirement has dropped from 595 Watts to 166 Watts.


Suggested Purchase List

The best brands as of August 2010 are Seasonic, Corsair, and the Antec NEO ECO series.

Note: all Antec PSUs do not ship with a power cable. All the power cables are the same, so you can take one from an old PSU or buy one from a local shop.

More Information