[Spread-users] New to spread and got some problems

Jeremy James jbj at forbidden.co.uk
Thu Jan 29 13:01:25 EST 2009

Hi John (Jeremy here, by the way),

> When node $x in segment $y sends a "Spread broadcast" it will first
> send a UDP point-to-point packet to the leaders of each of the other
> segments (who will then locally broad/multicast it within their segment)
> and then if it needs to broad/multicast within its segment it will do
> that too.

Ah, splendid. Leader determinted by config file ordering? So I should
make sure it is a less otherwise loaded machine towards the top? (or at
least preferably ones with faster NICs...)

> I would recommend that you use multicast addresses instead of
> broadcast addresses for your segments wherever it is available.
> Broadcast incurs CPU and switch bandwidth overhead on all machines /
> ports within the subnet whereas multicast can only use CPU and bandwidth
> on the machines / ports that have joined that IP multicast group.

Of course. And for multicast I should make sure I use different
(private) ranges for each cluster!

> If you can multicast between your two datacenters, then you should
> put
> them all in the same segment. If you can't multicast to all of them,
> then your .conf looks good (except you should use multicast within
> segments instead of broadcast).

As I say, I'd only be able to do a single multicast range with a
router-provided tunnel (GRE in this case - both ISPs use Cisco kit). But
I'll give unicast packets a go in the first case anyway.

> Make sure though that every daemon can send point-to-point traffic
> directly to every other daemon (i.e. - two distinct private (NAT'ed)
> networks usually can't work well together using Spread w/o some kind of
> tunnelling or default routing).

Sure - this wouldn't be a problem for the relevant hosts in my
datacentre scenario (as long as the firewalls are updated).


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