[Linux-HA] Availability Theory (STONITH)

[Alan Robertson]

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Robinson, Eric wrote:
>  
> Does attaching each of your cluster nodes to a separate NPS increase or
> decrease overall cluster availability? At first blush, I would think you
> want to eliminate the NPS as a SPOF, but I found a really interesting
> e-mail thread from 2002 where Lars seems to argue the opposite. It is a
> complex discussion and I'm not sure I follow it all. What is the current
> thinking on this subject? Should I plug both nodes of a 2-node cluster
> into the same NPS, or into 2 separate ones?
> 
> See:
> http://copilotconsulting.com/mail-archives/linux-ha.2002/msg01546.html

Doing exact computations on various failure methodologies depends on
knowing MTBF and MTTR measures of most of the components in the system.

It's really rare to actually know that information with any kind of
accuracy.

And, it depends a lot on the topology of the network involved as well,
which may in turn have unique failure modes.

For example, if you connected to each NPS with a dedicated NIC and
crossover cable, then the MTBF can be raised over that provided by a
"true network" with a switch and power supply - assuming that there is
no failure mode of direct connected equipment which doesn't occur with
switch-connected equipment.  [For some gigabit NICS and older buggy
drivers, there are such oddball unique failure modes -- sigh].


Of course, one potential problem -- if the NIC is only used for STONITH
- - it might fail without being noticed.

In Release 2, we monitor the connection to the STONITH device, so the
logs would have messages about the connection to the switch failing.



- --
    Alan Robertson <alanr at unix.sh>

"Openness is the foundation and preservative of friendship...  Let me
claim from you at all times your undisguised opinions." - William
Wilberforce
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