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================
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Merging clusters
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================
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With ``cluster-merge`` from the ``tools`` directory it is possible to
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merge two or more clusters into one single cluster.
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If anything goes wrong at any point the script suggests you rollback
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steps you've to perform *manually* if there are any. The point of no
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return is when the master daemon is started the first time after merging
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the configuration files. A rollback at this point would involve a lot of
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manual work.
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For the internal design of this tool have a look at the `Automated
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Ganeti Cluster Merger <design-2.1.rst>` document.
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Merge Clusters
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==============
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The tool has to be invoked on the cluster you like to merge the other
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clusters into.
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The usage of ``cluster-merge`` is as follows::
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  cluster-merge [--debug|--verbose] [--watcher-pause-period SECONDS] \
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  [--groups [merge|rename]] <cluster> [<cluster...>]
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You can provide multiple clusters. The tool will then go over every
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cluster in serial and perform the steps to merge it into the invoking
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cluster.
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These options can be used to control the behaviour of the tool:
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``--debug``/``--verbose``
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  These options are mutually exclusive and increase the level of output
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  to either debug output or just more verbose output like action
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  performed right now.
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``--watcher-pause-period``
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  Define the period of time in seconds the watcher shall be disabled,
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  default is 1800 seconds (30 minutes).
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``--groups``
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  This option controls how ``cluster-merge`` handles duplicate node
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  group names on the merging clusters. If ``merge`` is specified then
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  all node groups with the same name will be merged into one. If
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  ``rename`` is specified, then conflicting node groups on the remove
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  clusters will have their cluster name appended to the group name. If
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  this option is not speicifed, then ``cluster-merge`` will refuse to
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  continue if it finds conflicting group names, otherwise it will
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  proceed as normal.
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Rollback
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========
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If for any reason something in the merge doesn't work the way it should
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``cluster-merge`` will abort, provide an error message and optionally
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rollback steps. Please be aware that after a certain point there's no
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easy way to rollback the cluster to its previous state. If you've
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reached that point the tool will not provide any rollback steps.
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If you end up with rollback steps, please perform them before invoking
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the tool again. It doesn't keep state over invokations.
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