[Linux-HA] 2 subnets separated by cisco 6509 switch

ebay2 ebay2 at readynote.com
Tue Mar 15 20:50:11 MST 2005


I will investigate Cisco-Vlan options to get both boxes on the same subnet.
Thanks again.
-JPruett

>
>
> --On Tuesday, March 15, 2005 22:09:27 +0100 Kenneth Geisshirt
> <kenneth at geisshirt.dk> wrote:
>
> > Carson Gaspar wrote:
> >
> >> This really is a solved problem, but it does require a quorum
mechanism.
> >
> > A SAN connection over 20 km? Or a distributed/mirrored SAN with show
> > quorums like RHCS? I don't think it's a solved problem - maybe a partial
> > solved problem.
>
> No. This has _nothing_ to do with SANs.
>
> You just need an odd number of servers >= 3. At that point you have one of
> the following states for an active/standby config (all of this assumes
some
> sane quorum protocol - several exist):
>
> - Active and Standby servers can talk to each other
> - No problem
> - Active and Standby servers can't talk to each other, Active server can
> reach a quorum (>50% of servers)
> - Active server stays active. Standby server, by definition, can not reach
> a quorum, and thus may _not_ become active under any circumstance short of
> human override.
> - Active and Standby servers can't talk to each other, Active server can
> not reach a quorum
> - The Active server must give up all resources
> - The Standby server may acquire resources after some safety interval iff
> it has a quorum. The interval depends on how often the Active server
checks
> quorum, how long it takes to shut down resources, and how much of a safety
> margin you want
> - If the Standby server _also_ has no quorum it may not take over
> resources (so the service stays down)
>
> You have now solved the "who has control" problem. Replicate your data via
> SRDF, DRBD, or whatever, you won't have an unintentional active/active
> disaster (assuming properly behaved servers - STONITH has the extra
> guarantee of protecting against buggy servers).
>
> There is a _lot_ of literature about this - it seems sometimes that
> heartbeat is reinventing the wheel (from an algorithm/theory perspective).
>
> --
> Carson
>
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