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Cause:
User tries to stop all cluster nodes using 'pcs cluster stop --all'
Consequence:
Resources are moved from node to node pointlessly as all nodes will eventually be stopped. Cluster looses quorum as soon as enough nodes has stopped.
Fix:
Stop nodes in parallel to prevent services being moved. Stop pacemaker on all the nodes, wait for it to finish, then stop corosync on the nodes to keep the cluster quorate as long as possible.
Result:
Services are not moved around the cluster. Quorum is lost at a time when pacemaker and all resources has been stopped.
DescriptionFabio Massimo Di Nitto
2015-01-09 10:22:43 UTC
when issuing pcs cluster stop --all the stop sequence will:
stop all services on node1 -> stop cluster on node1
stop all services on node2 -> stop cluster on node2
....
There are 2 major problems with approach:
1) at somepoint there will be loss of quorum that can potentially block some resources that need quorum to stop (so when you get to the N node the resource will fail to stop and node will kill itself)
2) some applications (for example galera) have their own internal quorum and needs to be stopped all together to avoid corner case data corruption. Generally we can assume (for now at least) that external application quorum follows our quorum rules.
What we see in OpenStack is that stop --all on 3 node, will result in:
- moving tons of services around unnecessarily (since they will all die sooner or later)
- node fencing when stopping the last node because services can´t write to the db anymore (lack of internal quorum)
My recommendation here is:
on pcs cluster stop --all -> stop all resources -> stop cluster (at this point it´s irrelevant if it´s done one at a time or all together)
on pcs cluster stop -> this operation should verify if removing a node from the cluster will cause loss of quorum and abort. Of course we want to allow a manual override --pretty-please-i-know-i-will-lose-quorum-go-on
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHBA-2015-0415.html