I don't know the answer to this, but i suspect one takes the full load with the other effectively on standby. I might have the opportunity to test this theory, may take a few weeks before I can though.
Can I ask why you want to know?
If your datacenter is set up correctly, the answer is irrelevant.
The reason I'm trying to figure this out is that each circuit that comes into each of the racks is rated at finite amount of power, in this case, each PDU is rated at 20 Amps. On one we're running 13 Amps, and another 9 Amps. We ran into an issue with cascading power failure, which appears to have been caused by one circuit failing, and I assume all the 'shared' load on the PSU all went to one PDU (22 Amps) it tripped that one causing total power failure.
I'm trying to determine how the power is distributed on the PSU's to the PDU's in order to gain a better understanding of it's workings. It would also be useful for future planning in lieu of just guessing what is needed and hoping it all works out.
Unfortunately the datacenter is setup correctly based on our requirements, but we've either outgrown it or we underestimated it. It was here before me, and I'm just trying to help resolve it. :)
If one power circuit fails, the other has to carry the whole load, so at any time each power circuit should be ready to take the full load at any time. Otherwise it wouldn't be redundant...
So build your power requirements based on the ability to run all powersupplies at their full rated power.
As already stated properly set up each of you PDUs need to be able to provide the total load of the system as dual supplies is not just to protect against an individual PSU failure but for any failure in the supply circuit.
To answer your question however it is very difficult to make 2 PSUs share the power equally as it would require the PSUs to be a matched pair which would not be achievable in normal production. Very high end systems do have this load sharing function but not at this level as each PSU would have to sense what load the other is delivering and adjust its output to take more load or release load as required. This may not be done by the PSU but by the control cct they both connect to.
I did my own experiments on this and what I found was that whichever PSU came on 1st took the most load. I was mixing around with some x9xx dual PSU units and a pair of APC's trying to get the load meter leds to match but a soon as they restarted I lost the balance. Triple PSUs are even more fun as most systems should only allow one of the three to go off line as a single PSU is not enough on its own.
I even read one document that advised that with dual PSUs only one should be fed from a UPS and the other direct from a clean AC supply so the System Manager can detect if there has been an AC failure.
BLARGH!@# hehe.. seems noone can give me just a straight answer on this. It's driving me crazy. I don't care if I have a 100 Amp circuit with only one PSU redundant 1850.. I just want to know how it works. :P
Last night we had another incident at the colo, where one of our PDU's flipped. Techs checked, and while the one PDU was offline, it was pulling 0 amps (duh) and the other was pulling 12.x amps.. then when he turned it back on, that PDU was pulling 9 amps, and the other was still at 12.x amps. This makes it more confusing! Well kinda.
I'm going to order a couple of those kill-a-watt things, and do some of my own testing. :) Thanks for everyone's input so far.
snapohead
1.2K Posts
0
July 31st, 2008 02:00
I don't know the answer to this, but i suspect one takes the full load with the other effectively on standby. I might have the opportunity to test this theory, may take a few weeks before I can though.
Can I ask why you want to know?
If your datacenter is set up correctly, the answer is irrelevant.
jsurles
3 Posts
0
July 31st, 2008 03:00
The reason I'm trying to figure this out is that each circuit that comes into each of the racks is rated at finite amount of power, in this case, each PDU is rated at 20 Amps. On one we're running 13 Amps, and another 9 Amps. We ran into an issue with cascading power failure, which appears to have been caused by one circuit failing, and I assume all the 'shared' load on the PSU all went to one PDU (22 Amps) it tripped that one causing total power failure.
I'm trying to determine how the power is distributed on the PSU's to the PDU's in order to gain a better understanding of it's workings. It would also be useful for future planning in lieu of just guessing what is needed and hoping it all works out.
Unfortunately the datacenter is setup correctly based on our requirements, but we've either outgrown it or we underestimated it. It was here before me, and I'm just trying to help resolve it. :)
Dev Mgr
4 Operator
•
9.3K Posts
0
July 31st, 2008 11:00
So build your power requirements based on the ability to run all powersupplies at their full rated power.
AlanKS
241 Posts
0
August 3rd, 2008 20:00
As already stated properly set up each of you PDUs need to be able to provide the total load of the system as dual supplies is not just to protect against an individual PSU failure but for any failure in the supply circuit.
To answer your question however it is very difficult to make 2 PSUs share the power equally as it would require the PSUs to be a matched pair which would not be achievable in normal production. Very high end systems do have this load sharing function but not at this level as each PSU would have to sense what load the other is delivering and adjust its output to take more load or release load as required. This may not be done by the PSU but by the control cct they both connect to.
I did my own experiments on this and what I found was that whichever PSU came on 1st took the most load. I was mixing around with some x9xx dual PSU units and a pair of APC's trying to get the load meter leds to match but a soon as they restarted I lost the balance. Triple PSUs are even more fun as most systems should only allow one of the three to go off line as a single PSU is not enough on its own.
I even read one document that advised that with dual PSUs only one should be fed from a UPS and the other direct from a clean AC supply so the System Manager can detect if there has been an AC failure.
jsurles
3 Posts
0
August 5th, 2008 12:00
BLARGH!@# hehe.. seems noone can give me just a straight answer on this. It's driving me crazy. I don't care if I have a 100 Amp circuit with only one PSU redundant 1850.. I just want to know how it works. :P
Last night we had another incident at the colo, where one of our PDU's flipped. Techs checked, and while the one PDU was offline, it was pulling 0 amps (duh) and the other was pulling 12.x amps.. then when he turned it back on, that PDU was pulling 9 amps, and the other was still at 12.x amps. This makes it more confusing! Well kinda.
I'm going to order a couple of those kill-a-watt things, and do some of my own testing. :) Thanks for everyone's input so far.