[ClusterLabs] Questions about SBD behavior

Klaus Wenninger kwenning at redhat.com
Mon May 28 03:47:31 EDT 2018

On 05/28/2018 09:43 AM, Klaus Wenninger wrote:
> On 05/26/2018 07:23 AM, Andrei Borzenkov wrote:
>> 25.05.2018 14:44, Klaus Wenninger пишет:
>>> On 05/25/2018 12:44 PM, Andrei Borzenkov wrote:
>>>> On Fri, May 25, 2018 at 10:08 AM, Klaus Wenninger <kwenning at redhat.com> wrote:
>>>>> On 05/25/2018 07:31 AM, 井上 和徳 wrote:
>>>>>> Hi,
>>>>>> I am checking the watchdog function of SBD (without shared block-device).
>>>>>> In a two-node cluster, if one cluster is stopped, watchdog is triggered on the remaining node.
>>>>>> Is this the designed behavior?
>>>>> SBD without a shared block-device doesn't really make sense on
>>>>> a two-node cluster.
>>>>> The basic idea is - e.g. in a case of a networking problem -
>>>>> that a cluster splits up in a quorate and a non-quorate partition.
>>>>> The quorate partition stays over while SBD guarantees a
>>>>> reliable watchdog-based self-fencing of the non-quorate partition
>>>>> within a defined timeout.
>>>> Does it require no-quorum-policy=suicide or it decides completely
>>>> independently? I.e. would it fire also with no-quorum-policy=ignore?
>>> Finally it will in any case. But no-quorum-policy decides how
>>> long this will take. In case of suicide the inquisitor will immediately
>>> stop tickling the watchdog. In all other cases the pacemaker-servant
>>> will stop pinging the inquisitor which will makes the servant
>>> timeout after a default of 4 seconds and then the inquisitor will
>>> stop tickling the watchdog.
>>> But that is just relevant if Corosync doesn't have 2-node enabled.
>>> See the comment below for that case.
>>>>> This idea of course doesn't work with just 2 nodes.
>>>>> Taking quorum info from the 2-node feature of corosync (automatically
>>>>> switching on wait-for-all) doesn't help in this case but instead
>>>>> would lead to split-brain.
>>>> So what you are saying is that SBD ignores quorum information from
>>>> corosync and takes its own decisions based on pure count of nodes. Do
>>>> I understand it correctly?
>>> Yes, but that is just true for this case where Corosync has 2-node
>>> enabled.
>>>> In all other cases (might it be clusters with more than 2 nodes
>>> or clusters with just 2 nodes but without 2-node enabled in
>>> Corosync) pacemaker-servant takes quorum-info from
>>> pacemaker, which will probably come directly from Corosync
>>> nowadays.
>>> But as said if 2-node is configured with Corosync everything
>>> is different: The node-counting is then actually done
>>> by the cluster-servant and this one will stop pinging the
>>> inquisitor (instead of the pacemaker-servant) if it doesn't
>>> count more than 1 node.
>> Is it conditional on having no shared device or it just checks two_node
>> value? If it always behaves this way, even with real shared device
>> present, it means sbd is fundamentally incompatible with two_node and it
>> better be mentioned in documentation.
> If you are referring to counting the nodes instead of taking
> quorum-info from pacemaker in case of 2-node configured
> with corosync, that is universal.
> And actually the reason why it is there is to be able to use
> sbd with a single disk on 2-nodes having 2-node enabled.
> Imagine quorum-info from corosync/pacemaker being used
> in that case:
> Image a cluster (node-a & node-b). node-a looses connection
> to the network and to the shared storage. node-a will still
> receive positive quorum from corosync as it has seen the other
> node already (since it is up). This will make it ignore the
> loss of the disk (survive on pacemaker).
> node-b is quoruate as well, sees the disk, uses the disk to
> fence node-a and will after a timeout assume node-a to be
> down -> split-brain.
Seeing the disk will prevent the reboot if that is what
was missing for you.
>>> That all said I've just realized that setting 2-node in Corosync
>>> shouldn't really be dangerous anymore although it doesn't make
>>> the cluster especially useful either in case of SBD without disk(s).
>>> Regards,
>>> Klaus
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