R.G. Keen's Blog

Staggered Disk Spin-up for Home Servers

Posted in Opensolaris Server, Power Considerations by rgkeen on May 2, 2010

You can spin up your disks one at a time if your hardware allows it. Making your hardware allow it can be tricky, but worthwhile. You can use a much smaller power supply.

Low power is a virtue in a home server, just like it is for a commercial site server. When a disk spins up, it temporarily uses as much as five to ten times as much power as it does when it’s operating normally.

That presents a problem when you turn on the power supply. I have eight disks in my server at the moment, six 750GB 3.5″ devices and two 40GB 2.5″ devices.  The main array is the six 3.5″ devices, which get most of their power from +12V. At start up they can pull as much as 3A (36W) of +12V power each. That’s 18A of +12V – 216W.  After spinning up, the disks settle to 6W each, or three amps of +12V for the whole array.

The rest of the system uses +12V to power the processor, memory, and other things, so booting the entire system takes even more. When the system gets past this power-on-peak, it settles to about 100W total. There is a big pulse of power to get the mess started.

No problem, just buy a big power supply right? Actually, that is a problem. Power supplies waste power making the power you use. The power supply efficiency is the ratio of the power you get out of it to the power you put into it from the AC power line. It’s only recently that you could get power supplies which stated, and lately, certify their efficiency. The best are rated for “80-plus” which stands for “better than 80 percent”. These especially-good power supplies eat 100W of AC power line power for every 80W of power they deliver to your computer.

What that doesn’t tell you is that the losses are usually fixed, or at least do not decrease much with lower power out.  A 100W-rated 80+ power supply “eats” 125W from the AC power line to make 100W of  output power, the 25W being the efficiency penalty. A 500W rated 80+ power supply wastes five times as much, 125W, to make 500W, and it probably wastes 100W all the time, even if the load is only getting 100W. And the 80-plus units are the good guys. Lower power supply efficiencies only make the wasted power worse.

There is therefore a big plus in terms of less wasted electricity and heat generation if you can buy a smaller power supply for your server. But we already figured out that we have to buy a power supply to withstand the power-on peak in power needs. The power supply probably loafs after that.

Staggered spin-up is one trick to getting the peak power down, letting you buy a smaller, less wasteful power supply. If you could power up only one disk at a time, you could start your system with a much smaller power supply. In my system, the measured idle power from the AC line is about 105W. Each disk pulls 6W at idle, but 36W at spinup.

Theoretically, I could get the system to start up successfully with a power supply rated at 105W plus 30W for the peak for one disk drive to spin up if i could ensure that only one disk would start at a time.  A 150W power supply would be fine for that! Not only cheaper, more economical to run and “greener”.

To get to staggered spin up, you have to enable both hardware and software to make it happen. Opensolaris is great for that, because you can enable staggered spinup. But how about the hardware?

It turns out that some SATA drives (notably from WD) have a jumper on the drive for enabling and disabling staggered spin up. Others (notably Seagate) use pin 11 of the SATA power connector for enabling staggered spinup. Disabling staggered spin up happens when pin 11 is grounded, enabling happens when it’s open. It took me a few days to find that out.

When I could not make staggered spin up work, I dug in and found out that my power supply cable connectors have pin 11 internally connected to pins 10 and 12 – they are hard wired to make Seagate drives not do staggered spin up! The only way to make Seagates do staggered spin up was to commit surgery on either the disks or the power connectors to isolate pin 11. Then they spun up only when the software told them to.

As usual, some of the more useful bits are buried quite deeply. But if you look, you can sift out some power and money savings in your server.

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2 Responses

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  1. Krey said, on September 7, 2012 at 5:57 pm

    I try to isolate pin 11 for some Seagate drives including ST3000DM001. Nothing happening. My Old WD with jumper start queued on one MB and did not start on another MB. Seagate start on both MB.

    • rgkeen said, on September 10, 2012 at 9:19 am

      From what little information I have found, there seems to be a large variation in both the ability to do staggered spin-up and the support for it on various motherboards. Also, there is a kind of ‘genetic drift’ where each year’s new motherboards change features to follow what’s selling well in the market based on AMD and Intel hardware releases.

      From this I conclude that the ability to do things like staggered spin-up will continue to be something which requires research and experimentation on each system that gets set up.


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