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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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Old 12-06-2017
Stilez Stilez is offline
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Default PSU recommendation for file server with 20+ HDDs and >average baseboard current draw

I decided to de-lurk to ask for a PSU recommendation.

My first heavy duty PSU an oversize [URL="http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=142"]Antec CP850[/URL (I had to angle-grind a case to fit it), and last years' were an Supernova 750 + 850 pair. All rated at the top for performance, ripple and build, and all work fine. I'm having trouble choosing my next PSU, so I'm asking instead of just looking for the few that get 9.5 - 10 in the reviews.

When I got the Supernova G2 850, it was to power a new file server with maybe 6-8 HDDs. Instead I'm on 16 HDDs + 2 SATA SSDs and likely to increase. I need the PSU to handle up to 20 - 22 HDDs (maybe 24 with margin for safety although 22 might be enough), 4 SSDs, the baseboard and PCIe cards, and "some headroom so I don't push it above 80% load", and I think the Supernova might be at risk of being close to its limit at peak use. Time to upgrade it! Probably to 1000 or more likely, 1200.

As it's a file server array, the disks will tend to all be idle together, or all busy or peak together. I expect idle (or at most low-ish use) 95%+ of the time, but when these disks get peak usage, they can draw up to 12V @ 3A + 5V @ 1.5A from the datasheets, so I have to consider power adequacy. Because cables can melt, I also need to think about power distribution (how many drives chained on each cable, how many PSU plugs for HDD cables, and voltage drop/other issues if over-chained). As a slight complicating factor, about half the drives are SAS, which use MOLEX not SATA. So I'm having to buy SATA-to-MOLEX adapters for the cables since the PSU comes with more SATA than MOLEX outputs.

The baseboard has above average draw as well. Baseboard current draw covers Supermicro + Intel onboard components (the board is set to high stability/higher power draw settings), 4 core Xeon v3 @ 3.5 - 4.0 GHz (likely to upgrade to similar but 6 core v4 at some time), 8 x 2400 ECC RDIMM modules, and on the PCIe bus, 2 x Chelsio 10 GB NIC cards (4 optical ports), 3 LSI 9211 HBAs, and an Intel P3700 NMVe card (up to 25W in use), and about 8 fans (2 CPU/chassis, 6 high air pressure Silverstones for the HDDs). None of the PCIe cards can take a PCIe auxiliary cable, they're purely baseboard powered. PSU+chassis cooling isn't an issue, it will have plenty of space + fans.

I'm not sure what amperage I need for this lot, plus headroom so it's not "at risk" at peak load. It probably needs to be pretty good efficiency in order to minimise power waste for the majority of the time when the load is much lower, and my personal preference is for very good ripple, regulation and build.

My other issue once I choose a PSU, is how best to distribute the current to the HDDs. Assuming SATA/MOLEX cables typically have up to 4 connectors and you can usually get away with 1 or 2 splitters, and assuming I discount the SSDs load for PSU chaining purposes (it's much smaller than HDDs), I would still need 5 or so separate SATA/MOLEX cables/connectors, and I'd feel more at ease with 6 - but as I said, PSUs of that size tend to be configured for 4 HDD cables with the other plugs being for extra CPU/PCIe/graphics cards, not SATA/MOLEX.

One option that's easy is to mod an extra SATA/MOLEX cable from a PCIe cable (voltmeter, check wire voltages, cut off near the plug and solder a SATA/MOLEX chain onto it instead of the PCIe outlet), and that might in fact be the simple and direct solution, to avoid having to buy a PSU that's over specced just to get the right ports. Provided it's safe to do so - I can't think why it wouldn't be. Maybe at most I'd need a single rail supply to be sure there's no misfit between HDD and PCIe current limit specs across the rails?
  • What sort of spec should I go for?
  • Am I right that it's safe to feed HDDs from the PCIe feed if there aren't enough plugs for them, by hacking together a SATA/MOLEX chain onto a PCIe header (assuming I get the correct voltages attached for each wire!)

Thank you for any thoughts you can give.
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Last edited by Stilez; 12-06-2017 at 01:59 PM.
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Old 12-06-2017
GI_Joe GI_Joe is offline
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650W are more than enough with staggered spin up. Just use Molex splitters accordingly to "distribute" the staggered spin-up on your cables = A cable with 4 Molex + 4 splitters = 8 Molex on 1 cable, so dont power those 8 up at the same time, but connect the splitters to another power-up phase of your drives.
Just pick a PSU with 2 CPU connectors to be flexible for future upgrades and try to find one with as many drive cables as possible. Like a 4 cable with 3 SATA/PATA connectors is is better than 3 cables with 4, reduces the total current on the cable

Long ago, we used 400W to power an 18 drive array at a trade show
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