How much power do I need to feed all the HD in my UnRAID?

[McFly]

If the regular UnRAID server is filled up with harddrives, how much power does it need?

I know the drives need more when they spin up, but how much?

 

If I would fill my UnRAID with the maximum amount of disks (I guess 16 disks), how much power would I need?

I think I will probably start with a PSU that is good but not great (one that won´t be able to feed all disks), since I won´t need all that power to begin with.

 

I will probably start with something like a 650W PSU now and then later on add another PSU when I add more disks (instead of buying a monster PSU right now), but I have no idea how many disks this first PSU can handle without any problems. When do I need to add another PSU?

 

Or is it better to buy a monster PSU directly?

[Billped]

Good question.

 

My unraid with five drives, measured a few minutes ago, uses 142W at idle and 180W when all five drives are spinning (after being read).  This means my drives are using ~8W each.  I have read that spin up can use twice the wattage so round up to 20W each at startup.  To confirm this, I shut down my system and restarted it.  Peak power consumption (about five seconds into the restart) was 288W.  (288-142)/5=29.  Hmmm, higher than I was expecting.  Of course, there are other devices being spun up in my system that may be skewing this, including a "far more powerful than needed" graphics card and fans that crank to max at powerup. OK, let's bump up the per drive number to 25W. If you have 16 drives, that is 400W of drives plus whatever your bare system uses (142W in my case) for a total of almost 550W.

 

Note: power consumption is being measured by my handy Zalman device which has been lab tested with less than 10% error.

 

There are two problems with using this kind of back-of-the-envelope analysis:

 

1. XYZ Watt power supplies do not put out that much power at real-world temps and the dropoff can be 30% or so

2. The drives use the 12W rail(s) of the PSU and, if the PSU load is skewed toward the 12V as it will be in a storage server, will fall even further short of its rated number.

 

At a minimum, I would suggest that your plan for a 650W PS would work, but I would scale the number of drives slowly so you can determine when a second unit may be needed (i.e. when the system becomes unstable).

 

BTW, two smaller PSUs are often a lot cheaper than one beast (i.e. the 1KW PSUs are like $250).  Perhaps you can find a screaming bargain on a pair of 400W supplies.

 

 

Bill

[McFly]

Hmmm, interesting. I never thought that a 650W psu would be enough, but apparently it isn´t way too low.

I guess it makes sense starting with a lower rated PSU.

 

How about two 550W units (or 500W)? One with the system and 4 drives (would peak at about 260W), and one for 12 drives (would peak at 300W).

That should be a easy task for the 550W units, I prefer to keep them with a bit of overkill capacity, I imagine they will last longer if never stressed out to the maximum capacity.

 

Sounds OK to me, I will plan accordingly (if nobody else comes with better suggestions) and begin with one 550W unit for my server.

[McFly]

I think it is better to go with 2 psu sompared to one.

They will probably be cheaper. I can probably spread the power-up to 2 "hits" for my UPS. First power up the hard drives on the "hard drive only"-psu, and then power up the system and the rest of the drives.

 

However, how to power up the drives on the drive-only psu... I have no idea. I guess I need to shortcut some wires on the motherboard connector, so the PSU thinks the motherboard wants power or something...

[Billped]

Hmmm, interesting. I never thought that a 650W psu would be enough, but apparently it isn´t way too low.

 

One very high quality 650W may be enough, but most of us buy second-tier PSUs because the price is so much better.  I picked up a good (but not great) 600W PSU for $20 (after rebate) - I couldn't turn that down.  If I start having power issues, I'll pick up a second unit.

 

 

Bill