Statistics
| Branch: | Tag: | Revision:

root / doc / design-multi-reloc.rst @ 4055b109

History | View | Annotate | Download (2.9 kB)

1
====================================
2
Moving instances accross node groups
3
====================================
4

    
5
This design document explains the changes needed in Ganeti to perform
6
instance moves across node groups. Reader familiarity with the following
7
existing documents is advised:
8

    
9
- :doc:`Current IAllocator specification <iallocator>`
10
- :doc:`Shared storage model in 2.3+ <design-shared-storage>`
11

    
12
Motivation and and design proposal
13
==================================
14

    
15
At the moment, moving instances away from their primary or secondary
16
nodes with the ``relocate`` and ``multi-evacuate`` IAllocator calls
17
restricts target nodes to those on the same node group. This ensures a
18
mobility domain is never crossed, and allows normal operation of each
19
node group to be confined within itself.
20

    
21
It is desirable, however, to have a way of moving instances across node
22
groups so that, for example, it is possible to move a set of instances
23
to another group for policy reasons, or completely empty a given group
24
to perform maintenance operations.
25

    
26
To implement this, we propose a new ``multi-relocate`` IAllocator call
27
that will be able to compute inter-group instance moves, taking into
28
account mobility domains as appropriate. The interface proposed below
29
should be enough to cover the use cases mentioned above.
30

    
31
Detailed design
32
===============
33

    
34
We introduce a new ``multi-relocate`` IAllocator call whose input will
35
be a list of instances to move, and a "mode of operation" that will
36
determine what groups will be candidates to receive the new instances.
37

    
38
The mode of operation will be one of:
39

    
40
- *Stay in group*: the instances will be moved off their current nodes,
41
  but will stay in the same group; this is what the ``relocate`` call
42
  does, but here it can act on multiple instances. (Typically, the
43
  source nodes will be marked as drained, to avoid just exchanging
44
  instances among them.)
45

    
46
- *Change group*: this mode accepts one extra parameter, a list of node
47
  group UUIDs; the instances will be moved away from their current
48
  group, to any of the groups in this list. If the list is empty, the
49
  request is, simply, "change group": the instances are placed in any
50
  group but their original one.
51

    
52
- *Any*: for each instance, any group is valid, including its current
53
  one.
54

    
55
In all modes, the groups' ``alloc_policy`` attribute will be honored.
56

    
57
Result
58
------
59

    
60
In all storage models, an inter-group move can be modeled as a sequence
61
of **replace secondary** and **failover** operations (when shared
62
storage is used, they will all be failover operations within the
63
corresponding mobility domain). This will be represented as a list of
64
``(instance, [operations])`` pairs.
65

    
66
For replace secondary operations, a new secondary node must be
67
specified. For failover operations, a node *may* be specified when
68
necessary, e.g. when shared storage is in use and there's no designated
69
secondary for the instance.
70

    
71
.. vim: set textwidth=72 :
72
.. Local Variables:
73
.. mode: rst
74
.. fill-column: 72
75
.. End: