Placement groups

In the previous section, we described what objects in Ceph are and how we can access them. Ceph distributes objects pseudo-randomly to storage devices based on predefined data assignment policies. The assignment policy uses a novel placement algorithm called CRUSH to choose a probabilistically balanced distribution of objects throughout the cluster. This ensures that devices are presented with a nearly uniform storage and workload distribution for most use-cases. When a cluster is expanded by adding new storage or old/failed devices are removed, it is necessary to restore balance by migrating data to restore the configured replication policies. Since this data movement is kept a minimum, augmenting an existing cluster is ...

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