[Linux-cluster] Protocol compatibility of DLM/Corosync across versions

Jean-Marc Saffroy saffroy at gmail.com
Tue Dec 20 01:56:51 UTC 2016

On Mon, 19 Dec 2016, David Teigland wrote:

> On Sun, Dec 18, 2016 at 08:46:25PM +0100, Jean-Marc Saffroy wrote:
> > Hi (again),
> > 
> > Another question I have regarding DLM and Corosync (because Corosync is 
> > required to use DLM): should I expect compatibility across versions?
> > 
> > I did a quick test between distributions running different kernels (CentOS 
> > 6, Centos7 and Ubuntu 14) but rather close versions of Corosync, and that 
> > test worked, but I am not sure if that was just luck. ;)
> I can only speak for the dlm part of that.  Between different
> distributions, I'd call it luck :) 

Ah, so that could be a serious problem for me. I hoped to be able to use 
dlm across distributions without having to qualify each possible 

> Within the context of one distribution things shouldn't break if the 
> distribution is doing it's job.

Does that mean that, for example, I could expect dlm instances in RHEL6 
and RHEL7 kernels to work together?

> Upstream, with no distribution context, I'm certainly aware of when 
> compatibility breaks between dlm_controld and corosync and between 
> different dlm_controld versions on nodes.  I try to avoid it, but there 
> are unpredictable reasons that it can break.

How could instances of dlm_controld interact badly? I thought they were 
just glue between dlm and corosync, and never directly talk on the 
network. Do they have network-visible side effects on dlm/corosync?

In the end, I need to work across distributions and their kernels, but I 
could build from source a specific version of corosync and (the userland 
part of) dlm. I expect that the kernel interface to dlm is stable 
(right?), so the biggest risk would be incompatibilities in the dlm 
protocol on the network. Is this protocol stable? With git I see that 
DLM_HEADER_MAJOR/MINOR macros changed very rarely in recent years but I 
can't tell if this is a good indicator.


saffroy at gmail.com

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