Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 47 insertions, 0 deletions

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+=========================
+ Data Placement Overview
+=========================
+
+Ceph stores, replicates, and rebalances data objects across a RADOS cluster
+dynamically. Because different users store objects in different pools for
+different purposes on many OSDs, Ceph operations require a certain amount of
+data- placement planning. The main data-placement planning concepts in Ceph
+include:
+
+- **Pools:** Ceph stores data within pools, which are logical groups used for
+  storing objects. Pools manage the number of placement groups, the number of
+  replicas, and the CRUSH rule for the pool. To store data in a pool, it is
+  necessary to be an authenticated user with permissions for the pool. Ceph is
+  able to make snapshots of pools. For additional details, see `Pools`_.
+
+- **Placement Groups:** Ceph maps objects to placement groups. Placement
+  groups (PGs) are shards or fragments of a logical object pool that place
+  objects as a group into OSDs. Placement groups reduce the amount of
+  per-object metadata that is necessary for Ceph to store the data in OSDs. A
+  greater number of placement groups (for example, 100 PGs per OSD as compared
+  with 50 PGs per OSD) leads to better balancing. For additional details, see
+  :ref:`placement groups`.
+
+- **CRUSH Maps:**  CRUSH plays a major role in allowing Ceph to scale while
+  avoiding certain pitfalls, such as performance bottlenecks, limitations to
+  scalability, and single points of failure. CRUSH maps provide the physical
+  topology of the cluster to the CRUSH algorithm, so that it can determine both
+  (1) where the data for an object and its replicas should be stored and (2)
+  how to store that data across failure domains so as to improve data safety.
+  For additional details, see `CRUSH Maps`_.
+
+- **Balancer:** The balancer is a feature that automatically optimizes the
+  distribution of placement groups across devices in order to achieve a
+  balanced data distribution, in order to maximize the amount of data that can
+  be stored in the cluster, and in order to evenly distribute the workload
+  across OSDs.
+
+It is possible to use the default values for each of the above components.
+Default values are recommended for a test cluster's initial setup. However,
+when planning a large Ceph cluster, values should be customized for
+data-placement operations with reference to the different roles played by
+pools, placement groups, and CRUSH.
+
+.. _Pools: ../pools
+.. _CRUSH Maps: ../crush-map
+.. _Balancer: ../balancer