[AusNOG] Question about hardware spec for a DC

[Colin Stubbs]

I once thought like that too.

But if you're re-organising power and yanking cables out and plugging them
back in on anything other than a lab system you should be working within a
declared maintenance window anyway.

There's too much risk that something will come out that shouldn't, or that
something will shift just enough to cause a problem.

It might be acceptable to do it without a window for a UAT/test/dev system
but anything revenue generating or revenue supporting... hell no.

Having the convenience and ability to re-arrange power cables at any time
of day without risk and loss of $'s stems from designing systems to cope
with failure to start with.

That may in small part be because you put two PSU's in the boxes; but it
won't be the only reason.

-Colin

On 24 April 2014 11:30, Paul Gear <ausnog at libertysys.com.au> wrote:

>  In my judgement, the big benefit of dual power supplies (particularly on
> servers, switches, & routers) is that it enables reorganising of power
> circuits in racks without requiring a downtime window.  Like you say, a
> comprehensive view of high availability means designing N + 1 redundancy at
> the system level, but dual power supplies to those N + 1 devices is a big
> convenience factor.
>
> Paul
>
>
> On 04/24/2014 11:25 AM, Colin Stubbs wrote:
>
> IMHO
>
> Avoid that auto power-transfer stuff in rack if you can. Those devices are
> best used only for low end boxes like NTU's/etc with which you can only
> ever install/utilise a single box at a time.
>
>  Buy equipment which has dual PSU's as an option in preference, but don't
> mandate it or mandate buying the two power supplies. Mandating it will just
> mean you're wasting part of your budget unnecessarily on every single
> purchase as you may force yourself to buy bigger boxes than required;
> and/or buy more PSU's than you need.
>
>  Avoid *depending* on dual PSU's if you can, e.g. don't design anything
> with the assumption having two power supplies in a box will keep everything
> working if there is a loss of power or if one of the PSU's fails.
>
>  Design for failure with N+1 redundancy at a system level, e.g. install
> two or more of every box and use them in active/active or active/standby
> capacities. If you do that you won't necessarily require two power supplies
> in each to achieve a very high level of availability.
>
>  Choose to use dual PSU's primarily based on the location and the power
> infrastructure available, in combination with how many boxes you have. e.g.
> if you're in a crappy DC where they can't deliver access to two genuinely
> independent sources of power the value of having two PSU's is greatly
> reduced regardless of how many many boxes you've installed.
>
>  Read up on HA concepts if you're not sure what you need or why.
>
>  13 years old now but this book is still handy and the concepts still
> hold true,
>
>
> http://www.ciscopress.com/store/high-availability-network-fundamentals-9781587130175
>
>
>
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