[Linux-cluster] qdisk WITHOUT fencing

[Kaloyan Kovachev]

On Mon, 21 Jun 2010 10:20:34 +0100, Gordan Bobic <gordan at bobich.net>
wrote:
> On 06/21/2010 08:52 AM, Kaloyan Kovachev wrote:
>> On Fri, 18 Jun 2010 18:15:09 +0200, brem belguebli
>> <brem.belguebli at gmail.com>  wrote:
>>> How do you deal with fencing when the intersite interconnects (SAN and
>>> LAN) are the cause of the failure ?
>>>
>>
>> GPRS or the good old modem over a phone line?
> 
> That isn't going to work if the whole site is down for whatever reason 
> (unlikely as it may be).
> 

If the whole site is down because of a power failure - yes (well, then you
don't need to actually fence anything) , but if the failure is just in the
intersite connection - alternative low speed connection to simply fence the
remote nodes and tell the remote SAN to block it's access should be enough.

> To protect yourself from the 100% outage of a remote site, the only sane

> way I of approaching it I can think of is to do something like the 
> following:
> 
> 1) Make each node fence itself off from the failed node using iptables 
> or some other firewalling method. The SAN should also be prevented from 
> allowing the booted out node back onto it.
> 

then each node should do that kind of fencing, but if a single node blocks
the port(s) on the switch (to the remote location) should be easier to do
as fencing agent. Again having additional communication channel will help -
"if it's just the link, then fence the remote nodes and don't block the
port(s)" this would avoid manual intervention to restore the link after the
outage is fixed

> 2) Fail over the IP address or DNS name of the service. Since it's 
> across different sites, you are likely to have to use something like RIP

> to re-route the IPs, so DNS on short refresh may well be an easier and 
> possibly safer option. It'll mean some downtime, but probably less than 
> any manual intervention in an unplanned case.
> 
> It's not entirely ideal, bit it's about as good as it is likely to get. 
> And you can write a fencing agent to do something like this easily
enough.
> 
> Gordan
> 
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