[Pacemaker] Pacemaker cluster: OpenAis communication channels

[Steven Dake]

You can run with one NIC (and switch) but then your NIC and switch
become a SPOF (single point of failure).  Vehicles have a spare tire for
a reason :)  If a NIC fails it may be ok to switch a service to a
different node.  If a switch fails, The entire cluster becomes disabled
until the switch returns to operation.

Availability is a mathematical equation: 

A = MTTF / (MTTF+MTTR)

Pacemaker improves availability (A) by reducing mean time to repair
(MTTR) using failover while keeping the mean time to failure (MTTF)
essentially the same (although it is generally a bit lower because of
other components in the system required to introduce redundancy).
Instead of a typical 1 machine MTTR of 4 hours under a typical SLA, MTTR
may be 5-10 seconds or less (the time to failover the application and
restart it).  If MTTR is several days to service a switch, your
availability may not meet your customer SLA obligations.  When
determining whether to use a redundant switch the risks vs cost have to
be evaluated based upon your availability requirements.

Regards
-steve

On Thu, 2009-10-22 at 09:56 +0800, Romain CHANU wrote:
> Hi,
> 
> 
> 
> I am reading the user's guide for DRBD and there is something I want
> to clarify about Pacemaker.
> 
> 
> 
> In Chapter 8 ("Integrating DRBD with Pacemaker clusters"), section
> 8.1.4 "OpenAis communication channels", it is said that "the absolute
> minimum requirement for stable cluster operation is two independent
> communication channels in a redundant ring"
> 
> 
> Does it mean that if I have a Pacemaker cluster composed of two nodes,
> I need two NIC on each node?
> 
> 
> Thank you.
> 
> 
> Romain Chanu
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