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- opresult: OpCodes results as a list

For a successful opcode, the ``opresult`` field corresponding to it will

contain the raw result from its :term:`LogicalUnit`. In case an opcode

has failed, its element in the opresult list will be a list of two

elements:

- first element the error type (the Ganeti internal error name)

- second element a list of either one or two elements:

  - the first element is the textual error description

  - the second element, if any, will hold an error classification

The error classification is most useful for the ``OpPrereqError``

error type - these errors happen before the OpCode has started

executing, so it's possible to retry the OpCode without side

effects. But whether it make sense to retry depends on the error

classification:

``resolver_error``

  Resolver errors. This usually means that a name doesn't exist in DNS,

  so if it's a case of slow DNS propagation the operation can be retried

  later.

``insufficient_resources``

  Not enough resources (iallocator failure, disk space, memory,

  etc.). If the resources on the cluster increase, the operation might

  succeed.

``wrong_input``

  Wrong arguments (at syntax level). The operation will not ever be

  accepted unless the arguments change.

``wrong_state``

  Wrong entity state. For example, live migration has been requested for

  a down instance, or instance creation on an offline node. The

  operation can be retried once the resource has changed state.

``unknown_entity``

  Entity not found. For example, information has been requested for an

  unknown instance.

``already_exists``

  Entity already exists. For example, instance creation has been

  requested for an already-existing instance.

``resource_not_unique``

  Resource not unique (e.g. MAC or IP duplication).

``internal_error``

  Internal cluster error. For example, a node is unreachable but not set

  offline, or the ganeti node daemons are not working, etc. A

  ``gnt-cluster verify`` should be run.

``environment_error``

  Environment error (e.g. node disk error). A ``gnt-cluster verify``

  should be run.

Note that in the above list, by entity we refer to a node or instance,

while by a resource we refer to an instance's disk, or NIC, etc.