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38 lines
1.9 KiB
ReStructuredText
=========================
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Data Placement Overview
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=========================
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Ceph stores, replicates and rebalances data objects across a RADOS cluster
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dynamically. With many different users storing objects in different pools for
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different purposes on countless OSDs, Ceph operations require some data
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placement planning. The main data placement planning concepts in Ceph include:
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- **Pools:** Ceph stores data within pools, which are logical groups for storing
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objects. Pools manage the number of placement groups, the number of replicas,
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and the ruleset for the pool. To store data in a pool, you must have
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an authenticated user with permissions for the pool. Ceph can snapshot pools.
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See `Pools`_ for additional details.
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- **Placement Groups:** Ceph maps objects to placement groups (PGs).
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Placement groups (PGs) are shards or fragments of a logical object pool
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that place objects as a group into OSDs. Placement groups reduce the amount
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of per-object metadata when Ceph stores the data in OSDs. A larger number of
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placement groups (e.g., 100 per OSD) leads to better balancing. See
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`Placement Groups`_ for additional details.
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- **CRUSH Maps:** CRUSH is a big part of what allows Ceph to scale without
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performance bottlenecks, without limitations to scalability, and without a
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single point of failure. CRUSH maps provide the physical topology of the
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cluster to the CRUSH algorithm to determine where the data for an object
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and its replicas should be stored, and how to do so across failure domains
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for added data safety among other things. See `CRUSH Maps`_ for additional
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details.
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When you initially set up a test cluster, you can use the default values. Once
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you begin planning for a large Ceph cluster, refer to pools, placement groups
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and CRUSH for data placement operations.
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.. _Pools: ../pools
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.. _Placement Groups: ../placement-groups
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.. _CRUSH Maps: ../crush-map
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