Synnefo » snf-ganeti » ganeti-local

  gnt-group add <group> # add a new node group

  gnt-group del <group> # delete an empty node group

  gnt-group list # list node groups

  gnt-group rename <oldname> <newname> # rename a node group

  gnt-node {list,info} -g <group> # list only nodes belonging to a node group

  gnt-node modify -g <group> # assign a node to a node group

Node group attributes

+++++++++++++++++++++

In clusters with more than one node group, it may be desirable to

establish local policies regarding which groups should be preferred when

performing allocation of new instances, or inter-group instance migrations.

To help with this, we will provide an ``alloc_policy`` attribute for

node groups. Such attribute will be honored by iallocator plugins when

making automatic decisions regarding instance placement.

The ``alloc_policy`` attribute can have the following values:

- unallocable: the node group should not be a candidate for instance

  allocations, and the operation should fail if only groups in this

  state could be found that would satisfy the requirements.

- last_resort: the node group should not be used for instance

  allocations, unless this would be the only way to have the operation

  succeed.

- preferred: the node group can be used freely for allocation of

  instances (this is the default state for newly created node

  groups). Note that prioritization among groups in this state will be

  deferred to the  iallocator plugin that's being used.

Node group operations

+++++++++++++++++++++

One operation at the node group level will be initially provided::

  gnt-group drain <group>

The purpose of this operation is to migrate all instances in a given

node group to other groups in the cluster, e.g. to reclaim capacity if

there are enough free resources in other node groups that share a

storage pool with the evacuated group.

With the introduction of node groups, instances will be required to live

in only one group at a time; this is mostly important for DRBD

instances, which will not be allowed to have their primary and secondary

nodes in different node groups. To support this, we envision the

following changes:

  - The iallocator interface will be augmented, and node groups exposed,

    so that plugins will be able to make a decision regarding the group

    in which to place a new instance. By default, all node groups will

    be considered, but it will be possible to include a list of groups

    in the creation job, in which case the plugin will limit itself to

    considering those; in both cases, the ``alloc_policy`` attribute

    will be honored.

  - If, on the other hand, a primary and secondary nodes are specified

    for a new instance, they will be required to be on the same node

  - Cluster verify will signal an error if an instance has nodes

    belonging to different groups. Additionally, changing the group of a

    given node will be initially only allowed if the node is empty, as a

    straightforward mechanism to avoid creating such situation.

  - Inter-group instance migration will have the same operation modes as

    new instance allocation, defined above: letting an iallocator plugin

    decide the target group, possibly restricting the set of node groups

    to consider, or specifying a target primary and secondary nodes. In

    both cases, the target group or nodes must be able to accept the

    instance network- and storage-wise; the operation will fail

    otherwise, though in the future we may be able to allow some

    parameter to be changed together with the move (in the meantime, an

    import/export will be required in this scenario).

  - Frequent multinode operations, such as os-diagnose or cluster-verify,

    will act on one group at a time, which will have to be specified in

    all cases, except for clusters with just one group. Command line

    tools will also have a way to easily target all groups, by

    generating one job per group.

    be referenced by a UUID, which will be immutable; for example, nodes

    will contain the UUID of the group they belong to. This is done