Serial Port Concentrators vs. KVMs

Donald Becker becker at scyld.com
Fri Apr 25 12:43:35 EDT 2003


On Fri, 25 Apr 2003, Dan Kidger wrote:

> > On Thu, Apr 24, 2003 at 04:28:12PM +0100, Dan Kidger elucidated:
> > > However the current trend is for all new rack-mount nodes to offer some
> sort
> > > of BMC (baseboard management controller) with an ethernet connection. As
> > > well as giving remote power cycling, this should allow
> 'Serial_over_Lan" -
> >
> > Course the thing I wonder about that is then would seem to loose some
> > redundancy
...
> >  Course I guess if a switch blows up, there may
> > not be any particular need to access a node.

That's the key idea: by reducing the cable count, you reduce the number
of things that can wrong.  And if the communications to the node is
down, there is little point in having anything else working.

> You do not necessirly lose any reduncancy..
> Compaq nodes have a extra ethernet socket for the BMC (Hence
> serial_over_lan).
...
> Alternatively the Intel MoBo's like E7501 hijack the same physical ethernet
> socket for the BMC.

The different approach is determined by the Ethernet chip is in use.
A special NIC design is required to transparently piggyback management
traffic on the main network channel, and to continue to do so when the
main system is powered off.  The selection is pretty much limited to a
few 10/100 chips from 3Com and Intel.  If the system board uses a
gigabit NIC, the BMC has to have its own network connection.

An second Ethernet network is far less expensive, more reliable and
easier to diagnose than a KVM or serial setup.  While I prefer having
only two cables, power and one Cat5, rather than three, even three is no
comparison to the complexity of other solutions.

> This may lose a little redundancy (if for example a
> cable falls out), but halves the amount of cat5 cabling needed

This is a good example of how fewer cables doesn't lose redundancy, it
decreases the points of failure and increases the reliability.

-- 
Donald Becker				becker at scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

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