[Linux-cluster] Fencing Race Question

[Scott Becker]

To clarify, example #2 in the faq: redundant power feeds, redundant APCs 
- no single point of failure. Provides High availability but complicates 
fencing and race prevention.

This necessitates two power switch logins to successfully fence. I'll 
test by manually logging in to one device and tell luci to fence a node. 
With my manual login getting in the way, the fence operation should fail 
and turn off no ports and not prevent manual login to other apc, no 
matter which switch I manually log into. I haven't got a chance to test 
this yet and I wanted to tap into your wisdom to test my logic.

    thanks
    scottb


Chris Harms wrote:
> We have our APC power strips group the 2 ports together. The fence 
> agent should login to only one IP and turn off the grouped ports 
> instead of logging in to each unit separately to turn off power.
>
>
> Scott Becker wrote:
>> In the FAQ, Fencing Questions
>> 13. What's the right way to configure fencing when I have redundant 
>> power supplies?
>>
>> I'm going to setup the second example.
>> My concern is about a race condition with the two devices:
>>
>> |<device name="pwr01" option="off" switch="1" port="1"/>
>> <device name="pwr02" option="off" switch="1" port="1"/>|
>>
>> If I had only one power switch, the first node to login successfully 
>> turns the other off (APC with one IP address). My concern is with two 
>> APCs, that whoever loses the race to login to APC #1 may win the race 
>> to login to APC #2 and then each will turn off only one of the 
>> other's power supplies.
>>
>> Is the fencing execution script designed to 1) perform all necessary 
>> fencing device logins successfully then and only then 2) issue the 
>> poweroff commands? Thereby averting a potential race because the 
>> loser of the race for APC #1 will give up and hand over device #2.
>>
>>     thanks
>>     newby
>>     scottb
>>
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