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Learning Ceph - Second Edition by Karan Singh, Vaibhav Bhembre, Anthony D'Atri

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PG Up and Acting sets

Each PG has an attribute called Acting Set, comprising the current primary OSD and presently active replicas. This set of OSDs is responsible for actively serving I/O at the given moment in time. When the cluster topology changes, a PG may be updated to span a different set of OSDs. This new set of OSDs might or might not overlap with the previous set and is known as the Up Set of each PG. The OSDs in the Up Set are responsible for transferring data over from the current Acting Set's OSDs because they are now responsible for the PG that was adjusted. Once the data is current on all OSDs in the Up Set, the Acting Set will be the same as the Up Set and the PG should be in an active state.

Thus these sets are stable and ...

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