Re: How do redundant power supplies in servers work?

On Sat, Sep 10, 2011 at 8:01 PM, Frank Bulk <[email protected]> wrote:
> How do redundant power supplies in servers work?  Do they "load-balance" the
> draw between both of them, or is one always drawing much more and the other
> idling?  And if they do load-balance, how fast is the switch-over time and
> is it done on the AC or DC side?

My experience is mostly with HP DL-series gear. In the current DL
lineup, you can select how you want the power supplies to be
configured - in one of three ways:

1. Odd active, Even standby
2. Even active, Odd standby
3. Load balanced

These types of power supplies are typically most efficient when run as
close to their rated capacity as possible, so we run ours in
active/passive mode, making sure that the active power supplies are
load-balanced between the A and B PDUs in our racks.

Regarding switchover time, I don't have any hard numbers other than to
state that they failover quick enough to not disrupt the server's
activity. My hunch is that the active power supply has enough reserve
power left in its capacitors that it can coast without utility power
for the few milliseconds it takes to activate the standby power
supply.

The failover is done on the DC side - a much simpler task to
accomplish than needing to worry about phase alignment, etc. on the AC
side.

-Erik
-- 
Eat sushi frequently. - Avi
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