[Linux-cluster] Some GDLM questions

[David Teigland]

> GDLM is not listed as a client of FENCE. This seems to imply
> that a GDLM application has to interact directly with FENCE to 
> deal with the unknown state problem in a 2 node cluster where each 
> member has 1 vote and expected votes is 1 (section 3.2.6.2, page 28)
> as otherwise the same lockspace could end up existing on multiple
> machines in a single cluster. How would an application interact
> with FENCE to prevent this or does this have to be handled by
> configuring the cluster to reboot in this case?

This is the quickest one to answer right off the bat.  We'll get to the others
over the next few days I expect.

Fencing is a service that runs on its own in a CMAN cluster; it's entirely
independent from other services.  GFS simply checks to verify fencing is
running before allowing a mount since it's especially dangerous for a mount to
succeed without it.

As soon as a node joins a fencing domain it will be fenced by another domain
member if it fails.  i.e. as soon as a node runs:

> cman_tool join    (joins the cluster)
> fence_tool join   (starts fenced which joins the default fence domain)

it will be fenced by other domain members if it fails.  So, you simply need to
configure your nodes to run fence_tool join after joining the cluster if you
want fencing to happen.  You can add any checks later on that you think are
necessary to be sure that the node is in the fence domain.  (Looking at
/proc/cluster/services is one way.)

Running fence_tool leave will remove a node cleanly from the fence domain (it
won't be fenced by other members.)

One note of warning.  If the fence daemon (fenced) process is killed on node X,
it appears to fenced processes on other nodes that X has left the domain
cleanly (just as if it had run fence_tool leave).  X only leaves the domain
"uncleanly" when the node itself fails (meaning the cluster manager decides X
has failed.)  There is some further development planned to address this.

-- 
Dave Teigland  <teigland at redhat.com>