[Pacemaker] CMAN, corosync & pacemaker

Leon Fauster leonfauster at googlemail.com
Thu Mar 21 10:28:17 EDT 2013

Am 21.03.2013 um 13:32 schrieb Ron Kerry <rkerry at sgi.com>:
> I believe I understand how to configure CMAN to work with corosync and pacemaker, but I do not really understand what roles these three packages all play. Perhaps this information is exposed in the written documentation that is available in various places, but I have not been able to discern it.

It seems you are in the same situation like me (last week) :-) So far i will 
provide my big picture. Deeper insights must be provided from the experts here.

> You can run the corosync/pacemaker (either ver=0 or ver=1) combination and get fully functioning HA clusters. What does adding CMAN to the mix add to the functionality? Or perhaps a better question, what functions does CMAN take over from either corosync or pacemaker and why is this considered a better combination? What are the deficiencies in corosync that running CMAN on top of it resolves?

AFAIK - the architecture should be seen mainly as a stack. On top of all is the so called 
"cluster resource manager (crm)" like pacemaker or rgmanager. Lets call it crm-layer and
we use pacemaker for it (check "pacemaker internals" in the documentation). 

Under the crm-layer is the communication layer (corosync) that checks e.g. cluster membership.

Membership is the keyword here when we take a look at cman - the quorum-concept handles 
the active cluster/members and this can be also done by pacemaker/corosync. cman can do
this (quorum handling) very well (mature code) and is doing this in the lower layers 
(extending corosync) - so thats the difference. When the lower level has some information 
(e.g. about the quorum status) then all upper levels have the same information. If pacemaker 
is doing this then they might be situations where different informations exists in the different 

> I believe the preferred pacemaker based HA configuration in RHEL 6.4 uses all three packages and the preferred configuration in SLES11 SP2 is just corosync/pacemaker (I do not believe CMAN is even available in SLE-HAE).
> Why the different approaches and what is the advantage of each configuration?

cman is part of the RH cluster suite - the motivations of the different ha approaches for 
both vendors are for sure more based in strategic/marketing needs.


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