Synnefo » snf-ganeti

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Moving instances accross node groups

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This design document explains the changes needed in Ganeti to perform

instance moves across node groups. Reader familiarity with the following

existing documents is advised:

- :doc:`Current IAllocator specification <iallocator>`

- :doc:`Shared storage model in 2.3+ <design-shared-storage>`

Motivation and and design proposal

==================================

At the moment, moving instances away from their primary or secondary

nodes with the ``relocate`` and ``multi-evacuate`` IAllocator calls

restricts target nodes to those on the same node group. This ensures a

mobility domain is never crossed, and allows normal operation of each

node group to be confined within itself.

It is desirable, however, to have a way of moving instances across node

groups so that, for example, it is possible to move a set of instances

to another group for policy reasons, or completely empty a given group

to perform maintenance operations.

To implement this, we propose a new ``multi-relocate`` IAllocator call

that will be able to compute inter-group instance moves, taking into

account mobility domains as appropriate. The interface proposed below

should be enough to cover the use cases mentioned above.

Detailed design

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We introduce a new ``multi-relocate`` IAllocator call whose input will

be a list of instances to move, and a "mode of operation" that will

determine what groups will be candidates to receive the new instances.

The mode of operation will be one of:

- *Stay in group*: the instances will be moved off their current nodes,

  but will stay in the same group; this is what the ``relocate`` call

  does, but here it can act on multiple instances. (Typically, the

  source nodes will be marked as drained, to avoid just exchanging

  instances among them.)

- *Change group*: this mode accepts one extra parameter, a list of node

  group UUIDs; the instances will be moved away from their current

  group, to any of the groups in this list. If the list is empty, the

  request is, simply, "change group": the instances are placed in any

  group but their original one.

- *Any*: for each instance, any group is valid, including its current

  one.

In all modes, the groups' ``alloc_policy`` attribute will be honored.

Result

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In all storage models, an inter-group move can be modeled as a sequence

of **replace secondary** and **failover** operations (when shared

storage is used, they will all be failover operations within the

corresponding mobility domain). This will be represented as a list of

``(instance, [operations])`` pairs.

For replace secondary operations, a new secondary node must be

specified. For failover operations, a node *may* be specified when

necessary, e.g. when shared storage is in use and there's no designated

secondary for the instance.

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