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ha-manager.adoc: improve section Recover Fenced Services

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Dietmar Maurer 2016-11-21 11:37:50 +01:00
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@ -575,20 +575,23 @@ the specified module at startup.
Recover Fenced Services
~~~~~~~~~~~~~~~~~~~~~~~
After a node failed and its fencing was successful we start to recover services
to other available nodes and restart them there so that they can provide service
again.
After a node failed and its fencing was successful, the CRM tries to
move services from the failed node to nodes which are still online.
The selection of the node on which the services gets recovered is influenced
by the users group settings, the currently active nodes and their respective
active service count.
First we build a set out of the intersection between user selected nodes and
available nodes. Then the subset with the highest priority of those nodes
gets chosen as possible nodes for recovery. We select the node with the
currently lowest active service count as a new node for the service.
That minimizes the possibility of an overload, which else could cause an
unresponsive node and as a result a chain reaction of node failures in the
cluster.
The selection of nodes, on which those services gets recovered, is
influenced by the resource `group` settings, the list of currently active
nodes, and their respective active service count.
The CRM first builds a set out of the intersection between user selected
nodes (from `group` setting) and available nodes. It then choose the
subset of nodes with the highest priority, and finally select the node
with the lowest active service count. This minimizes the possibility
of an overloaded node.
CAUTION: On node failure, the CRM distributes services to the
remaining nodes. This increase the service count on those nodes, and
can lead to high load, especially on small clusters. Please design
your cluster so that it can handle such worst case scenarios.
[[ha_manager_start_failure_policy]]