[Pacemaker] pingd

[Bernd Schubert]

On Friday, September 03, 2010, Lars Ellenberg wrote:
> > > how about an fping RA ?
> > > active=$(fping -a -i 5 -t 250 -B1 -r1 $host_list 2>/dev/null | wc -l)
> > > 
> > > terminates in about 3 seconds for a hostlist of 100 (on the LAN, 29 of
> > > which are alive).
> > 
> > Happy to add if someone writes it :-)
> 
> I thought so ;-)
> Additional note to whomever is going to:
> 
> With fping you can get fancy about "better connectivity",
> you are not limited to the measure "number of nodes responding".

I think for the beginning, just the basic feature should be sufficient. 
Actually I thought about to add an option to the existing ping RA to let the 
user choose between ping and fping, it would default to ping. I will do that 
mid of next week.


> You could also use the statistics on packet loss and rtt provided on
> stderr for -c or -C mode (example output below, chose what you think is
> easier to parse), then do some scoring scheme on average or max packet
> loss, rtt, or whatever else makes sense to you.
> (If a switch starts dying, it may produce increasing packet loss first...)

That will require quite parsing, which I'm not comfortable with in a shell 
script. I have no objections to later on add fping RA written in perl or 
python.

[...]

> 
> > >> PS: (*) As you insist ;) on quorum with n/2 + 1 nodes, we use ping as
> > >> replacement. We simply cannot fulfill n/2 + 1, as controller failure
> > >> takes down 50% of the systems (virtual machines) and the systems
> > >> (VMs) of the 2nd controller are then supposed to take over failed
> > >> services. I see that n/2 + 1 is optimal and also required for a few
> > >> nodes. But if you have a larger set of system (e.g. minimum 6 with
> > >> the VM systems I have in my mind) n/2 + 1 is sufficient, IMHO.
> > > 
> > > You meant to say you consider == n/2 sufficient, instead of > n/2 ?
> 
> So you have a two node virtualization stuff, each hosting n/2 VMs,
> and do the pacemaker clustering between those VMs?

Yes.

> 
> I'm sure you could easily add "somewhere else" a very bare bone VM
> (or real) server, that is dedicated member of your cluster, but
> never takes any resources? Just serves as arbitrator? as your "+1"?

No, I'm afraid it is not that easy. There is simply is nothing that can be 
used. If there is anything, it is always available on both hosts/controllers. 
Imagine you would sell a standalone DRBD system (black box), that provides for 
example NFS to clients. You would want to have each and every additional 
service mirrored again. And you could not rely on additional customer NFS 
clients.

> 
> May be easier, safer, and more transparent than
> no-quorum=ignore plus some ping attribute based auto-shutdown.

I agree on safer and transparent, but unfortunately, it not easier in our 
case.

-- 
Bernd Schubert
DataDirect Networks