Commit f5d723fe authored by Adeodato Simo's avatar Adeodato Simo Committed by Guido Trotter

Update 2.3 design doc regarding node group features and behavior.

In particular:

  - introduce a "gnt-group" command to hold group-level operations.
  - ditch the concept of "default node group", except for single-group
    clusters.
  - introduce an "alloc_policy" attribute for node groups, indicating
    how they should be treated by automated allocation tools.
  - introduce a "drain" operation on node groups.
  - define iallocator modes for new instance allocation and
    inter-group moves (choosing among all groups, or providing a
    limiting list).
  - indicate and explain that changing the group of a node will be
    initially only supported for nodes that are empty.
Signed-off-by: default avatarAdeodato Simo <dato@google.com>
Signed-off-by: default avatarBalazs Lecz <leczb@google.com>
Reviewed-by: default avatarGuido Trotter <ultrotter@google.com>
parent add6d208
......@@ -51,61 +51,102 @@ Node group management
To manage node groups and the nodes belonging to them, the following new
commands and flags will be introduced::
gnt-node group-add <group> # add a new node group
gnt-node group-del <group> # delete an empty group
gnt-node group-list # list node groups
gnt-node group-rename <oldname> <newname> # rename a group
gnt-node list/info -g <group> # list only nodes belonging to a group
gnt-node add -g <group> # add a node to a certain group
gnt-node modify -g <group> # move a node to a new group
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.
Instance level changes
++++++++++++++++++++++
Instances will be able to live in only one group at a time. This is
mostly important for DRBD instances, in which case both their primary
and secondary nodes will need to be in the same group. To support this
we envision the following changes:
- The cluster will have a default group, which will initially be
- Instance allocation will happen to the cluster's default group
(which will be changeable via ``gnt-cluster modify`` or RAPI) unless
a group is explicitly specified in the creation job (with -g or via
RAPI). Iallocator will be only passed the nodes belonging to that
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
group.
- Moving an instance between groups can only happen via an explicit
operation, which for example in the case of DRBD will work by
performing internally a replace-disks, a migration, and a second
replace-disks. It will be possible to clean up an interrupted
group-move operation.
- Cluster verify will signal an error if an instance has been left
mid-transition between groups.
- Inter-group instance migration/failover will check that the target
group will be able to accept the instance network/storage wise, and
fail otherwise. In the future we may be able to make some parameter
changed during the move, but in the first version we expect an
import/export if this is not possible.
- From an allocation point of view, inter-group movements will be
shown to a iallocator as a new allocation over the target group.
Only in a future version we may add allocator extensions to decide
which group the instance should be in. In the meantime we expect
Ganeti administrators to either put instances on different groups by
filling all groups first, or to have their own strategy based on the
instance needs.
- 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).
Internal changes
++++++++++++++++
We expect the following changes for cluster management:
- Frequent multinode operations, such as os-diagnose or cluster-verify
will act on one group at a time. The default group will be used if none
is passed. Command line tools will have a way to easily target all
groups, by generating one job per group.
- 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.
- Groups will have a human-readable name, but will internally always
be referenced by a UUID, which will be immutable. For example the
cluster object will contain the UUID of the default group, each node
will contain the UUID of the group it belongs to, etc. This is done
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
to simplify referencing while keeping it easy to handle renames and
movements. If we see that this works well, we'll transition other
config objects (instances, nodes) to the same model.
......@@ -483,9 +524,6 @@ Note that due to the way the tspecs are computed, for any given
specification, the total available count is the count for the given
entry, plus the sum of counts for higher specifications.
Also note that the node group information is provided just
informationally, not for allocation decisions.
Node flags
----------
......
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