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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Redundant power supplies
Date: Tue, 29 Oct 2024 23:33:49 -0700
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On 10/27/2024 8:00 PM, Don Y wrote:
> 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).

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!