Synnefo » snf-ganeti » ganeti-local

Beside the node role, there are other node flags that influence its

behaviour:

- the *master_capable* flag denotes whether the node can ever become a

  master candidate; setting this to 'no' means that auto-promotion will

  never make this node a master candidate; this flag can be useful for a

  remote node that only runs local instances, and having it become a

  master is impractical due to networking or other constraints

- the *vm_capable* flag denotes whether the node can host instances or

  not; for example, one might use a non-vm_capable node just as a master

  candidate, for configuration backups; setting this flag to no

  disallows placement of instances of this node, deactivates hypervisor

  and related checks on it (e.g. bridge checks, LVM check, etc.), and

  removes it from cluster capacity computations

Node flags

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Current state and shortcomings

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Currently all nodes are, from the point of view of their capabilities,

homogeneous. This means the cluster considers all nodes capable of

becoming master candidates, and of hosting instances.

This prevents some deployment scenarios: e.g. having a Ganeti instance

(in another cluster) be just a master candidate, in case all other

master candidates go down (but not, of course, host instances), or

having a node in a remote location just host instances but not become

master, etc.

Proposed changes

~~~~~~~~~~~~~~~~

Two new capability flags will be added to the node:

- master_capable, denoting whether the node can become a master

  candidate or master

- vm_capable, denoting whether the node can host instances

In terms of the other flags, master_capable is a stronger version of

"not master candidate", and vm_capable is a stronger version of

"drained".

For the master_capable flag, it will affect auto-promotion code and node

modifications.

The vm_capable flag will affect the iallocator protocol, capacity

calculations, node checks in cluster verify, and will interact in novel

ways with locking (unfortunately).

It is envisaged that most nodes will be both vm_capable and

master_capable, and just a few will have one of these flags

removed. Ganeti itself will allow clearing of both flags, even though

this doesn't make much sense currently.

      <title>Node flags</title>

      <para>Nodes have two flags which govern which roles they can take:

        <variablelist>

          <varlistentry>

            <term>master_capable</term>

            <listitem>

              <para>

                The node can become a master candidate, and

                furthermore the master node. When this flag is

                disabled, the node cannot become a candidate; this can

                be useful for special networking cases, or less

                reliable hardware.

              </para>

            </listitem>

          </varlistentry>

          <varlistentry>

            <term>vm_capable</term>

            <listitem>

              <para>

                The node can host instances. When enabled (the default

                state), the node will participate in instance

                allocation, capacity calculation, etc. When disabled,

                the node will be skipped in many cluster checks and

                operations.

              </para>

            </listitem>

          </varlistentry>

        </variablelist>

      </para>

    </refsect2>

    <refsect2>