[Linux-cluster] Fencing problem w/ 2-node VM when a VM host dies

Kelvin Edmison kelvin.edmison at alcatel-lucent.com
Fri Dec 4 18:52:06 UTC 2015

On 12/04/2015 12:49 PM, Digimer wrote:
> On 04/12/15 09:14 AM, Kelvin Edmison wrote:
>> On 12/03/2015 09:31 PM, Digimer wrote:
>>> On 03/12/15 08:39 PM, Kelvin Edmison wrote:
>>>> On 12/03/2015 06:14 PM, Digimer wrote:
>>>>> On 03/12/15 02:19 PM, Kelvin Edmison wrote:
>>>>>> I am hoping that someone can help me understand the problems I'm
>>>>>> having
>>>>>> with linux clustering for VMs.
>>>>>> I am clustering 2 VMs on two separate VM hosts, trying to ensure
>>>>>> that a
>>>>>> service is always available.  The hosts and guests are both RHEL 6.7.
>>>>>> The goal is to have only one of the two VMs running at a time.
>>>>>> The configuration works when we test/simulate VM deaths and
>>>>>> graceful VM
>>>>>> host shutdowns, and administrative switchovers (i.e. clusvcadm -r ).
>>>>>> However, when we simulate the sudden isolation of host A (e.g. ifdown
>>>>>> eth0), two things happen
>>>>>> 1) the VM on host B does not start, and repeated fence_xvm errors
>>>>>> appear
>>>>>> in the logs on host B
>>>>>> 2) when the 'failed' node is returned to service, the cman service on
>>>>>> host B dies.
>>>>> If the node's host is dead, then there is no way for the survivor to
>>>>> determine the state of the lost VM node. The cluster is not allowed to
>>>>> take "no answer" as confirmation of fence success.
>>>>> If your hosts have IPMI, then you could add fence_ipmilan as a backup
>>>>> method where, if fence_xvm fails, it moves on and reboots the host
>>>>> itself.
>>>> Thank you for the suggestion.  The hosts do have ipmi.  I'll explore it
>>>> but I'm a little concerned about what it means for the other
>>>> non-clustered VM workloads that exist on these two servers.
>>>> Do you have any thoughts as to why host B's cman process is dying when
>>>> 'host A' returns?
>>>> Thanks,
>>>>     Kelvin
>>> It's not dieing, it's blocking. When a node is lost, dlm blocks until
>>> fenced tells it that the fence was successful. If fenced can't contact
>>> the lost node's fence method(s), then it doesn't succeed and dlm stays
>>> blocked. To anything that uses DLM, like rgmanager, it appears like the
>>> host is hung but it is by design. The logic is that, as bad as it is to
>>> hang, it's better than risking a split-brain.
>> when I said the cman service is dying, I should have further qualified
>> it. I mean that the corosync process is no longer running (ps -ef | grep
>> corosync does not show it)  and after recovering the failed host A,
>> manual intervention (service cman start) was required on host B to
>> recover full cluster services.
>> [root at host2 ~]# for SERVICE in ricci fence_virtd cman rgmanager; do
>> printf "%-12s   " $SERVICE; service $SERVICE status; done
>> ricci          ricci (pid  5469) is running...
>> fence_virtd    fence_virtd (pid  4862) is running...
>> cman           Found stale pid file
>> rgmanager      rgmanager (pid  5366) is running...
>> Thanks,
>>    Kelvin
> Oh now that is interesting...
> You'll want input from Fabio, Chrissie or one of the other core devs, I
> suspect.
> If this is RHEL proper, can you open a rhbz ticket? If it's CentOS, and
> if you can reproduce it reliably, can you create a new thread with the
> reproducer?
It's RHEL proper in both host and guest, and we can reproduce it reliably.

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