Re: [Evms-cluster] Access to EVMS volumes with CSM with only one node alive

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Steve Dobbelstein wrote:
| Maximilian Wilhelm <[email protected]> wrote on 04/25/2008 01:15:48 PM:
|
|  > Hi!
|  >
|  > I've set up a Xen-Cluster with two nodes sharing some SAN storage.
|  >
|  > I decided to use EVMS to cut on of the LUNs into pieces for the DomU
|  > storage. So I put a CSM on the LUN devices, and some LVM stuff in
|  > there.
|  >
|  > If I now reboot/poweroff one node for any reason, the /dev/evms/...
|  > devices managent by CSM on the running node disappear, which is quite
|  > bad as I cannot start Xen DomUs when one node failed.
|  >
|  > Is this intentional or can I do anything about that?
|  >
|  > I can't imagine any reason for this behaviour and would realy like to
|  > be able to use the volumes if only one node is alive.
|  >
|  > Ciao
|  > Max
|  > --
|  >    Follow the white penguin.
|
| Hi, Max.
|
| The EVMS CSM will not display any shared storage on a node if that node
| is not in the membership of the cluster.  The reason is that if the node
| is not in the membership then the CSM cannot guarantee that the node can
| safely access the shared storage.
|
| Last I heard it was still undecided if a single node in a two node
| cluster can declare that it is part of the membership of the cluster.
|  All that the node can determine is that it cannot talk to the other
| node.  It doesn't know if it is the only one alive or if it lost its
| connection to the other node.  If the other node is alive, which one is
| in the membership and which one is not?  If both nodes are up but cannot
| talk to each other, neither one can declare that it is the one in the
| membership and that the other node isn't.  The typical way to handle the
| scenario is to have the node declare itself as not being in the
| membership.  This is the safe solution.  I have heard that there are
| some implementations or scenarios that allow one node to have then
| membership, but I don't recall the specifics.
|
| If you want to check if your node is in the membership, run "evmsccm
| -l".  The evmsccm command is used by EVMS to determine if a node is in
| the membership.  The "-l" switch says to list the nodes in the membership.

I'm using EVMS CSM's in a 2-node Linux-HA cluster (Linux-HA v2.x with v1
config) and I don't have any problem with single node. If one node
gracefully quits (/etc/init.d/heartbeat shutdown) the other node keeps
membership. If communication breaks the other node gets gets STONITH'ed
instead (using both serial & udp for communication).

Also if I start a node and it can't communicate with the other one it
will STONITH it before taking resources. For that reason I don't start
heartbeat on bootup to avoid possibility of broken communication causing
nodes to kill each other in a round-robin way :)

- --
Thomas
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