2e74ccd0b7
Signed-off-by: Rafael Jardim <rafaeljordao.jardim@windriver.com> Change-Id: Ic8eea41e912e52ddebc5ed9dca62e8d4f9255b09
86 lines
4.2 KiB
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86 lines
4.2 KiB
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.. awp1552678699112
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.. _replication-groups:
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==================
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Replication Groups
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==================
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The storage hosts on Ceph systems are organized into replication groups to
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provide redundancy.
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Each replication group contains a number of hosts, referred to as peers.
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Each peer independently replicates the same data. |prod| supports a minimum
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of two peers and a maximum of three peers per replication group. This
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replication factor is defined when the Ceph storage backend is added.
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For a system with two peers per replication group, up to four replication
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groups are supported. For a system with three peers per replication group,
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up to three replication groups are supported.
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For best performance, |org| recommends a balanced storage capacity, in
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which each peer has sufficient resources to meet the operational
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requirements of the system.
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A replication group is considered healthy when all its peers are available.
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When only a minimum number of peers are available \(as indicated by the
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**min\_replication** value reported for the group\), the group continues to
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provide storage services but without full replication, and a HEALTH\_WARN
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state is declared. When the number of available peers falls below the
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**min\_replication** value, the group no longer provides storage services,
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and a HEALTH\_ERR state is declared. The **min\_replication** value is
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always one less than the replication factor for the group.
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It is not possible to lock more than one peer at a time in a replication
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group.
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Replication groups are created automatically. When a new storage host is
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added and an incomplete replication group exists, the host is added to the
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existing group. If all existing replication groups are complete, then a new
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incomplete replication group is defined and the host becomes its first
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member.
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.. note::
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Ceph relies on monitoring to detect when to switch from a primary OSD
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to a replicated OSD. The Ceph parameter :command:`osd heartbeat grace` sets
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the amount of time required to wait before switching OSDs when the
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primary OSD is not responding. |prod| currently uses the default value
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of 20 seconds. This means that a Ceph filesystem may not respond to I/O
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for up to 20 seconds when a storage node or OSD goes out of service.
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For more information, see the Ceph documentation:
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`http://docs.ceph.com/docs/master/rados/configuration/mon-osd-interaction
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<http://docs.ceph.com/docs/master/rados/configuration/mon-osd-interaction>`__.
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Replication groups are shown on the Hosts Inventory page in association
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with the storage hosts. You can also use the following CLI commands to
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obtain information about replication groups:
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.. code-block:: none
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~(keystone_admin)]$ system cluster-list
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+--------------------------------------+--------------+------+----------+------------------+
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| uuid | cluster_uuid | type | name | deployment_model |
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+--------------------------------------+--------------+------+----------+------------------+
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| 335766eb-8564-4f4d-8665-681f73d13dfb | None | ceph | ceph_clu | controller-nodes |
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| | | | ster | |
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| | | | | |
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+--------------------------------------+--------------+------+----------+------------------+
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.. code-block:: none
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~(keystone_admin)]$ system cluster-show 335766eb-968e-44fc-9ca7-907f93c772a1
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+--------------------+----------------------------------------+
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| Property | Value |
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+--------------------+----------------------------------------+
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| uuid | 335766eb-968e-44fc-9ca7-907f93c772a1 |
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| cluster_uuid | None |
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| type | ceph |
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| name | ceph_cluster |
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| replication_groups | ["group-0:['storage-0', 'storage-1']"] |
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| storage_tiers | ['storage (in-use)'] |
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| deployment_model | controller-nodes |
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+--------------------+----------------------------------------+
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