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- the :command:`ganeti-masterd` daemon which runs on the master node and |
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allows control of the cluster |
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Beside the node role, there are other node flags that influence its |
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behaviour: |
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- the *master_capable* flag denotes whether the node can ever become a |
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master candidate; setting this to 'no' means that auto-promotion will |
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never make this node a master candidate; this flag can be useful for a |
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remote node that only runs local instances, and having it become a |
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master is impractical due to networking or other constraints |
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- the *vm_capable* flag denotes whether the node can host instances or |
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not; for example, one might use a non-vm_capable node just as a master |
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candidate, for configuration backups; setting this flag to no |
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disallows placement of instances of this node, deactivates hypervisor |
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and related checks on it (e.g. bridge checks, LVM check, etc.), and |
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removes it from cluster capacity computations |
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Instance |
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~~~~~~~~ |
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b/doc/design-2.3.rst | ||
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informationally, not for allocation decisions. |
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Node flags |
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---------- |
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Current state and shortcomings |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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Currently all nodes are, from the point of view of their capabilities, |
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homogeneous. This means the cluster considers all nodes capable of |
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becoming master candidates, and of hosting instances. |
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This prevents some deployment scenarios: e.g. having a Ganeti instance |
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(in another cluster) be just a master candidate, in case all other |
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master candidates go down (but not, of course, host instances), or |
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having a node in a remote location just host instances but not become |
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master, etc. |
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Proposed changes |
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~~~~~~~~~~~~~~~~ |
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Two new capability flags will be added to the node: |
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- master_capable, denoting whether the node can become a master |
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candidate or master |
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- vm_capable, denoting whether the node can host instances |
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In terms of the other flags, master_capable is a stronger version of |
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"not master candidate", and vm_capable is a stronger version of |
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"drained". |
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For the master_capable flag, it will affect auto-promotion code and node |
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modifications. |
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The vm_capable flag will affect the iallocator protocol, capacity |
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calculations, node checks in cluster verify, and will interact in novel |
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ways with locking (unfortunately). |
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It is envisaged that most nodes will be both vm_capable and |
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master_capable, and just a few will have one of these flags |
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removed. Ganeti itself will allow clearing of both flags, even though |
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this doesn't make much sense currently. |
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Job priorities |
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-------------- |
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</refsect2> |
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<refsect2> |
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<title>Node flags</title> |
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<para>Nodes have two flags which govern which roles they can take: |
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<variablelist> |
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<varlistentry> |
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<term>master_capable</term> |
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<listitem> |
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<para> |
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The node can become a master candidate, and |
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furthermore the master node. When this flag is |
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disabled, the node cannot become a candidate; this can |
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be useful for special networking cases, or less |
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reliable hardware. |
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</para> |
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</listitem> |
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</varlistentry> |
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<varlistentry> |
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<term>vm_capable</term> |
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<listitem> |
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<para> |
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The node can host instances. When enabled (the default |
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state), the node will participate in instance |
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allocation, capacity calculation, etc. When disabled, |
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the node will be skipped in many cluster checks and |
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operations. |
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</para> |
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</listitem> |
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</varlistentry> |
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</variablelist> |
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</para> |
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</refsect2> |
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<refsect2> |
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<title>Cluster configuration</title> |
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<para>The master node keeps and is responsible for the cluster |
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