[ClusterLabs] Questions about SBD behavior

Klaus Wenninger kwenning at redhat.com
Mon May 28 03:43:22 EDT 2018

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.

>> 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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