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Re: [Linux-cluster] DRBD with GFS applicable for this scenario?



On Thu, Jan 28, 2010 at 10:25 AM, Gordan Bobic <gordan bobich net> wrote:

> The problem you have is that you have no way of enacting fencing if the
> connectivity between the sites fails. If a node fails, any cluster file
> system (GFS included) will mandate a fencing action to ensure that one of
> the nodes gets taken down and stays down. If you have lost cross-site
> connectivity, the nodes won't be able to fence each other, and GFS will
> simply block until connectivity is restored and fencing succeeds. The
> chances are that when this happens, it'll also cause a fencing shoot-out and
> both nodes may well end up getting fenced.
>
> You could use some kind of cheat-fencing, say, by setting a firewall rule
> that will prevent the nodes from re-connecting (you'd need to write your own
> fencing agent, but that's not particularly difficult), but then you would be
> pretty much guaranteeing a split-brain situation, where the nodes would end
> up operating independently without any hope of ever re-synchronising.
>
> The bottom line is that you need reliable out-of-band fencing mechanism. If
> you have GSM/wireless signal in both areas you could rig up a separate,
> small fencing "server" on each site with a GSM modem, and write a fencing
> agent that sends a fencing request by SMS. When the fencing server receives
> a fencing request, you'd have to make it issue a local fencing action using
> one of the more standard fencing agents. Note that in this case, due to high
> latency of things like SMS, you'd need to implement accurate time stamping
> and deliberately semi-randomize the delay between fencing requests being
> sent so that you could check time stamps and the fencing servers could
> sensibly decide whether to obey the local fencing request or the remote one.
>
> You have to get a little creative about it and write a few lines of code to
> glue it together. I've been meaning to implement something like this for a
> while, but I haven't gotten around to it yet.
>


This seems to address the split brain condition that can occur in case
of network blackouts involving a 2-node cluster.

https://bugzilla.redhat.com/show_bug.cgi?id=372901


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