>> Apparently the settings have nothing (little?) to do
>> with reliability/redundancy. Rather, they are there
>> to improve energy efficiency (!)
>>
>> From the long list of settings he sent me, these people REALLY
>> try to save every watt they can! I guess if you have
>> thousands of servers, a few watts on each has consequences
>> (cooling, etc.).
>>
>> [As a quick test, I was able to change the power requirements
>> for my server addressing a fixed load by more than 10%!]
>
>After playing with assorted setting combinations, it
>is obvious that they do nothing to affect the (shortterm)
>availability of the server.
>
>When configured to share the load, the load is approximately
>split evenly between the two supplies -- if both are powered on.
>Unplug either and the other takes the full load.
>
>When configured as hot spare, select PS#1 as "primary", unplug
>that power cord and PS#2 "spins up" to take on the load.
>
>Unplug PS#2 and, as expected, no change (other than a
>warning indicating that you are now completely reliant on
>that ONE power supply).
>
>*BUT*, the total power consumed goes UP when the load
>is shared if the power supplies are "lightly" loaded.
>This makes sense as efficiency tends to improve with
>the magnitude of the load.
>
>In my case, there was about a 25W "penalty" for operating
>in the sharing configuration (on a ~220W load -- single CPU,
>external SAS HBA, 8x1TB, 256GB DRAM).
Nothing is free, but you will have double the hold-up time.
>Moral of story is to size individual power supply to handle
>the entire (projected/future) load and then use the hot spare
>to give you redundancy. So, this is an enhanced feature that
>isn't available in my other servers...
>
>[Amusing as I tend to oversize the power supplies to address
>unknown future needs... annoying to have to upgrade a power
>supply -- PAIR of power supplies -- just because you want to
>embelish the hardware in a box!]
>
>But, still no idea why I should be able to *select* the primary
>supply if both are cabled!
Not all redundant supplies are hot swappable - but rely on the supply being pulled/inserted to be in a disabled/dead or powered down state.
It's a connector pin sequencing and dynamic start-up/shutdown/ engagement performance issue.
RL