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

[Friedrich Weber <f.weber@proxmox.com>]

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"?

> +
> +* We *advise against* using bond modes *balance-rr*, *balance-xor*,
> +  *balance-tlb*, or *balance-alb* for Corosync traffic. As explained above,
> +  they can cause asymmetric connectivity in certain failure scenarios.
> +
> +* *IEEE 802.3ad (LACP)*: This bond mode can cause asymmetric connectivity in
> +  certain failure scenarios as explained above, but it can recover from this
> +  state, as each side can stop using a bonded interface if it has not received
> +  three LACPDUs in a row. However, with default settings, LACPDUs are only sent
> +  every 30 seconds, yielding a failover time of 90 seconds. This is too long,
> +  as nodes with HA resources will fence themselves already after roughly one
> +  minute without a stable quorum. If LACP bonds are used for corosync traffic,
> +  we recommend setting `bond-lacp-rate fast` *on the Proxmox VE node and the
> +  switch*! Setting this option on one side requests the other side to send an
> +  LACPDU every second. Setting this option on both sides can reduce the
> +  failover time in the scenario above to 3 seconds and thus prevent fencing.
> +
> +* Bond mode *active-backup* will not cause asymmetric connectivity in the
> +  failure scenario described above, but the affected node may lose connection
> +  to the cluster and, if HA is enabled, fence itself.
> +
>  Separate Cluster Network
>  ~~~~~~~~~~~~~~~~~~~~~~~~
>  



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