[Linux-cluster] N+M support ?

[Rafael Micó Miranda]

Hi Martin,

Al your questions point to an advanced configuration using Failover
Domains and Service propierties.

El sáb, 13-02-2010 a las 06:14 +0000, Martin Waite escribió:
> Hi,
>  
> Suppose I have 3 services running on 5 nodes.  Each node can run only 1 service, 2 nodes are reserved for failover.

Lets name the services S1, S2, S3, and the nodes N1, N2, N3 and N4, N5
for the failover ones.

>  
> It is easy to configure rgmanager to cope with the first service node failure by including the 2 failover nodes in the failover domain for each service.   

Yes, you can configure it in the following way (name the failover
domains F1, F2, F3)

F1: service S1, nodes N1, N4, N5
F2: service S2, nodes N2, N4, N5
F3: service S3, nodes N3, N4, N5

You'll need to set the failover domains with the properties "restricted"
and "ordered" for this. There is another property, "auto_failback", that
will be of your interest. Keep it in mind.

>  
> However, is it possible to configure rgmanager such that on a second failure, only the failover node that is not currently running a service is considered for use ?  

Yes. Services have the "run exclusive" option. That property will only
allow a service to be run on a node that has no running services. With
this option on, if N1 fails it will fail over the N4. After that if N2
fails the service S2 will be migrated to N5, and not to N4. 

>  
> Further, that if a third failure occurs, the affected service is not migrated at all ?

Yes. If you have set the "run exclusive" option on the three services
the service will not be migrated to any node according to:

http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Cluster_Administration/s1-add-service-CA.html


>  
> Also, is it possible to rank the services such that if the failover nodes are occupied by low ranking services and the node running a higher ranking service fails, that the lowest ranking service is evicted so that the higher ranking service can be failed over ?
>  

As far as I know, there is no direct function to do what you ask. 

You can play with all this options to get something similar to what you
need. For example I propose you this configuration:

Nodes:

N1, N2, N3: service nodes
N4, N5: failover nodes

Services:

S1: high ranking service. "Run exclusive" on.
S2, S3: low ranking services. "Run exclusive" off.

Fail Over Domains:

F1: service S1. Nodes N1, N4, N5. "Restricted" on. "Ordered on". 
"Auto_failback" off.
F2: service S2. Nodes N2, N5, N4. "Restricted" on. "Ordered on".
"Auto_failback" on.
F3: service S3. Nodes N3, N5, N4. "Restricted" on. "Ordered on".
"Auto_failback" on.

With this configuration you only penalise services S2 and S3 in case of
N2 and N3 failure because both services will run on the same node, but
you keep N4 free for your hing ranking S1 service. With "Auto_failback"
on on faiolver domains F2 and F3 you will automatically migrate S2 or S3
back to its preferred node when they come back alive, so penalisation
will be shorter in time.


Remember that you are talking about a failure of up to 3 nodes in a
cluster of 5 members. Maybe there is no sense in this because depending
on the configuration given you can even lose Quorum before achieving
this situation.


> regards,
> Martin
>  
> 
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Cheers,

Rafael

-- 
Rafael Micó Miranda