[pve-devel] [PATCH docs v2] pvecm, network: add section on corosync over bonds

[Friedrich Weber]

On 25/07/2025 14:23, Mira Limbeck wrote:
> 
> 
> On 7/25/25 13:50, Friedrich Weber wrote:
>> On 25/07/2025 13:39, Friedrich Weber wrote:
>>> [...]
>>> +Corosync Over Bonds
>>> +~~~~~~~~~~~~~~~~~~~
>>> +
>>> +Using a xref:sysadmin_network_bond[bond] as the only Corosync link can be
>>> +problematic in certain failure scenarios. If one of the bonded interfaces fails
>>> +and stops transmitting packets, but its link state stays up, some bond modes
>>> +may cause a state of asymmetric connectivity where cluster nodes can only
>>> +communicate with different subsets of other nodes. In case of asymmetric
>>> +connectivity, Corosync may not be able to form a stable quorum in the cluster.
>>> +If this state persists and HA is enabled, nodes may fence themselves, even if
>>> +their respective bond is still fully functioning. In the worst case, the whole
>>> +cluster may fence itself.
>>> +
>>> +For this reason, our recommendations are as follows.
>>> +
>>> +* We recommend a dedicated physical NIC for the primary Corosync link. Bonds
>>> +  can be used as additional links for increased redundancy.
>>
>> These recommendations are still not 100% clear: Are we fine with a setup
>> with
>>
>> - link 0: dedicated corosync link
>> - link 1: corosync link over a bond with a problematic mode (such as
>> balance-rr or LACP with bond-lacp-rate slow)
>>
>> ?
>> In my tests, as long as the dedicated link 0 is completely online, it
>> doesn't matter if a bond runs into the failure scenario above (one of
>> the bonded NICs stops transmitting packets), corosync will just continue
>> using link 0. But as soon as link 0 goes down and the failure scenario
>> happens, the whole-cluster fence may happen. So should our
>> recommendation be the relatively strict "if you put corosync on a bond
>> (even if it is only a redundant link), use only active-backup or
>> LACP+bond-lacp-rate fast"?
> 
> I'd say yes, the recommendation should be either dedicated link
> directly, or a bond as redundant link with active-backup or
> LACP+lacp-rate fast only.

Thanks for the input. I've rephrased the section (and did some other
adjustments) to make it clear that the caveats apply whenever a bond is
used for corosync traffic.

v3:
https://lore.proxmox.com/pve-devel/20250725140312.250936-1-f.weber@proxmox.com/T/