Synnefo » snf-ganeti

Node Groups

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Current state and shortcomings

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

Currently all nodes of a Ganeti cluster are considered as part of the

same pool, for allocation purposes: DRBD instances for example can be

allocated on any two nodes.

This does cause a problem in cases where nodes are not all equally

connected to each other. For example if a cluster is created over two

set of machines, each connected to its own switch, the internal bandwidth

between machines connected to the same switch might be bigger than the

bandwidth for inter-switch connections.

Moreover some operations inside a cluster require all nodes to be locked

together for inter-node consistency, and won't scale if we increase the

number of nodes to a few hundreds.

Proposed changes

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

With this change we'll divide Ganeti nodes into groups. Nothing will

change for clusters with only one node group, the default one. Bigger

cluster instead will be able to have more than one group, and each node

will belong to exactly one.

Node group management

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

To manage node groups and the nodes belonging to them, the following new

commands/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 belongin 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

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 changable via gnt-cluster modify or RAPI) unless a

    group is explicitely specified in the creation job (with -g or via

    RAPI). Iallocator will be only passed the nodes belonging to that

    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 cleanup an interrupted

    group-move operation.

  - Cluster verify will signal an error if an instance has been left

    mid-transition between groups.

  - Intra-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/Internal/Config level changes

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

We expect the following changes for cluster management:

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

    will act 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.

  - 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

    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.

  - The addition of a new per-group lock will be evaluated, if we can

    transition some operations now requiring the BGL to it.

  - Master candidate status will be allowed to be spread among groups.

    For the first version we won't add any restriction over how this is

    done, although in the future we may have a minimum number of master

    candidates which Ganeti will try to keep in each group, for example.

Other work and future changes

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

Commands like gnt-cluster command/copyfile will continue to work on the

whole cluster, but it will be possible to target one group only by

specifying it.

Commands which allow selection of sets of resources (for example

gnt-instance start/stop) will be able to select them by node group as

well.

Initially node groups won't be taggable objects, to simplify the first

implementation, but we expect this to be easy to add in a future version

should we see it's useful.

We envision groups as a good place to enhance cluster scalability. In

the future we may want to use them ad units for configuration diffusion,

to allow a better master scalability. For example it could be possible

to change some all-nodes RPCs to contact each group once, from the

master, and make one node in the group perform internal diffusion. We

won't implement this in the first version, but we'll evaluate it for the

future, if we see scalability problems on big multi-group clusters.

When Ganeti will support more storage models (eg. SANs, sheepdog, ceph)

we expect groups to be the basis for this, allowing for example a

different sheepdog/ceph cluster, or a different SAN to be connected to

each group. In some cases this will mean that inter-group move operation

will be necessarily performed with instance downtime, unless the

hypervisor has block-migrate functionality, and we implement support for

it (this would be theoretically possible, today, with KVM, for example).