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The CLI `status` method tells you what backend is selected, and also gives the backend an opportunity to complain if something seems wrong with its configuration. Signed-off-by: John Spray <john.spray@redhat.com>
165 lines
5.6 KiB
ReStructuredText
165 lines
5.6 KiB
ReStructuredText
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.. _orchestrator-modules:
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.. py:currentmodule:: orchestrator
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ceph-mgr orchestrator modules
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=============================
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.. warning::
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This is developer documentation, describing Ceph internals that
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are only relevant to people writing ceph-mgr orchestrator modules.
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In this context, *orchestrator* refers to some external service that
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provides the ability to discover devices and create Ceph services. This
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includes external projects such as ceph-ansible, DeepSea, and Rook.
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An *orchestrator module* is a ceph-mgr module (:ref:`mgr-module-dev`)
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which implements common managment operations using a particular
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orchestrator.
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Orchestrator modules subclass the ``Orchestrator`` class: this class is
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an interface, it only provides method definitions to be implemented
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by subclasses. The purpose of defining this common interface
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for different orchestrators is to enable common UI code, such as
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the dashboard, to work with various different backends.
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Behind all the abstraction, the purpose of orchestrator modules is simple:
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enable Ceph to do things like discover available hardware, create and
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destroy OSDs, and run MDS and RGW services.
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A tutorial is not included here: for full and concrete examples, see
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the existing implemented orchestrator modules in the Ceph source tree.
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Glossary
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--------
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Stateful service
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a daemon that uses local storage, such as OSD or mon.
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Stateless service
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a daemon that doesn't use any local storage, such
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as an MDS, RGW, nfs-ganesha, iSCSI gateway.
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Label
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arbitrary string tags that may be applied by administrators
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to nodes. Typically administrators use labels to indicate
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which nodes should run which kinds of service. Labels are
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advisory (from human input) and do not guarantee that nodes
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have particular physical capabilities.
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Drive group
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collection of block devices with common/shared OSD
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formatting (typically one or more SSDs acting as
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journals/dbs for a group of HDDs).
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Placement
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choice of which node is used to run a service.
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Key Concepts
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------------
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The underlying orchestrator remains the source of truth for information
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about whether a service is running, what is running where, which
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nodes are available, etc. Orchestrator modules should avoid taking
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any internal copies of this information, and read it directly from
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the orchestrator backend as much as possible.
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Bootstrapping nodes and adding them to the underlying orchestration
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system is outside the scope of Ceph's orchestrator interface. Ceph
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can only work on nodes when the orchestrator is already aware of them.
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Calls to orchestrator modules are all asynchronous, and return *completion*
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objects (see below) rather than returning values immediately.
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Where possible, placement of stateless services should be left up to the
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orchestrator.
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Completions and batching
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------------------------
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All methods that read or modify the state of the system can potentially
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be long running. To handle that, all such methods return a *completion*
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object (a *ReadCompletion* or a *WriteCompletion*). Orchestrator modules
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must implement the *wait* method: this takes a list of completions, and
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is responsible for checking if they're finished, and advancing the underlying
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operations as needed.
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Each orchestrator module implements its own underlying mechanisms
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for completions. This might involve running the underlying operations
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in threads, or batching the operations up before later executing
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in one go in the background. If implementing such a batching pattern, the
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module would do no work on any operation until it appeared in a list
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of completions passed into *wait*.
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*WriteCompletion* objects have a two-stage execution. First they become
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*persistent*, meaning that the write has made it to the orchestrator
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itself, and been persisted there (e.g. a manifest file has been updated).
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If ceph-mgr crashed at this point, the operation would still eventually take
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effect. Second, the completion becomes *effective*, meaning that the operation has really happened (e.g. a service has actually been started).
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.. automethod:: Orchestrator.wait
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.. autoclass:: ReadCompletion
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.. autoclass:: WriteCompletion
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Placement
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---------
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In general, stateless services do not require any specific placement
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rules, as they can run anywhere that sufficient system resources
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are available. However, some orchestrators may not include the
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functionality to choose a location in this way, so we can optionally
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specify a location when creating a stateless service.
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OSD services generally require a specific placement choice, as this
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will determine which storage devices are used.
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Excluded functionality
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----------------------
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- Ceph's orchestrator interface is not a general purpose framework for
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managing linux servers -- it is deliberately constrained to manage
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the Ceph cluster's services only.
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- Multipathed storage is not handled (multipathing is unnecessary for
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Ceph clusters). Each drive is assumed to be visible only on
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a single node.
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Inventory and status
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--------------------
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.. automethod:: Orchestrator.get_inventory
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.. autoclass:: InventoryFilter
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.. autoclass:: InventoryNode
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.. autoclass:: InventoryDevice
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.. automethod:: Orchestrator.describe_service
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.. autoclass:: ServiceDescription
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.. autoclass:: ServiceLocation
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OSD management
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--------------
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.. automethod:: Orchestrator.create_osds
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.. automethod:: Orchestrator.replace_osds
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.. automethod:: Orchestrator.remove_osds
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.. autoclass:: OsdCreationSpec
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.. autoclass:: DriveGroupSpec
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Upgrades
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--------
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.. automethod:: Orchestrator.upgrade_available
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.. automethod:: Orchestrator.upgrade_start
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.. automethod:: Orchestrator.upgrade_status
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.. autoclass:: UpgradeSpec
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.. autoclass:: UpgradeStatusSpec
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Utility
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-------
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.. automethod:: Orchestrator.available
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