What are symptoms of too little power?

[Marc G]

If you try to draw too much power from your power supply what happens? Will the system not boot? Would it boot but be more susceptible to system errors? Or would individual devices just fail what it can't get the power it needs?

 

The reason I ask is that according to one of those power calulators (you know, the kind that you input everything on your system that could possible draw power and it tells you how beefy your power supply needs to be) I should be able run my system with a 450w PS. I'm using one rated at 650w. The only reason I'm concerened is that I have more PATA drives and fans than the PS has 4-pin Molex connectors. Two 120mm fans and one drive require the use of y-adapters. From my research before, it seemed that this should be reasonable.

 

I just want to be sure.

 

Thanks,

Marc

[JRS]

That is a good question, especially with respect to an unRaid box.  Since unRaid is doing a specific task, psu problems may be more predictable than An unRaid box has most of its power supply demands when all the drives are spinning up at once, so maybe, and this is a complete guess, than if there are odd symptoms at bootup or when you spin up all drives at the same time, then it may be worthwhile to try a different the power supply.

 

IMHO, you should not be concerned about having more drives/fans than 4-pin molex.  I have no concerns running up to three disks and a fan off one PSU cable, as long as I would not have to daisy chain y-connectors to accomplish this.  My Antec earthwatts 380 has at least 3 connectors on each cable (either sata or molex) - I would be comfortable adding a Y-connector to the molex ones and running 3 drives and a fan off one of those.

 

I would think 650w should drive any 16 hard disks on unRaid.  A beefy unRaid box really deserves a quality PSU, such as PC Power & Cooling, Seasonic, Corsair, Antec earthwatts, etc.  One of those PSU's can exceed its rating on occassion without a problem, whereas a PSU from "psu's 'r us", will fail long before it gets close to drawing its rated wattage.  For the many companies, the rated wattage of a PSU is "marketing".  Some companies really stretch that marketing.

 

I'm hardly an expert on PSU's, so hopefully you'll get some input from someone who's a bit smarter in this area to reply.

[tillkrueger]

i have had PSU problems on my Powermac G5 after i put an additional 8 drives in a RAID5 configuration in it (on top of the two system drives)...the symptoms were that on a cold start (after having been turned off for a while), it would first turn on and then turn itself off again while all disks began to suck power at once...if i turned it back on almost immediately (while some disks were still spinning a bit), i would often be able to "trick" it into booting...however, quite frequently i had one or two disks in the array stop spinning due to power not being sufficient (i suppose), which - needless to say - is a realy pain in the ass in a RAID5...it would go into a critical state, if only for a couple of seconds, and then start a few hours of parity checks...i ended up having to use a second power-supply on top of my G5, running the molex cables inside and powering just the disks off that PSU...and that was just too messy for my taste, which is part of the reason why i just put together an unRAID system.

 

so, to answer your question more directly:

 

- random HD spin-downs

 

- it is unlikely that fans will fail, because they draw very little power.

 

- most problems will likely make themselves known during boot-up, as your system draws by far the most power.

 

- if you have a high-power processor and graphics card, then the other problems would rear their ugly head during game-play and rendering/transcoding (crashes and system shutdown)

 

that's my experience, anyway, although PSU problems have made up a disappearing small percentage of my problems in 20 years of professional computing.

JRS's advice to only buy from reputable manufacturers with conservative power-ratings is probably your best insurance against random shut-downs due to PSU problems.

 

[SSD]

- most problems will likely make themselves known during boot-up, as your system draws by far the most power.

Regular drives take about 30 watts to spin up, and about 10 to idle.  Green drives take about half that.  If your computer is booting up okay, you're probably fine.

 

In the old SCSI days, drives had jumpers that would allow you to delay boot by either 3 or 6 seconds.  Using these jumpers, you could control power consumption at boot.  Not sure why we don't have that today.

 

[Marc G]

Thanks for the input, guys.

[SSD]

I stumbled across the following article on PSUs.  It was an easy read and VERY informative.  Highly recommended.

 

http://www.playtool.com/pages/psucompat/compat.html

[limetech]

Overall quality is as important as overall power.  Many PSU power ratings are far more "generous" than what is actually measured.  Also cheap power supplies may have enough 'noise' or spikes on the power lines to cause intermittent failures such as DMA errors, lost interrupts, etc.

 

For a server use only a high-quality PSU with single +12V rail.