Ceph Placement Groups peering

The Peering state machine ( based on the Boost Statechart Library ) is in charge of making sure the OSDs supporting the placement groups are available, as described in the high level description of the peering process. It will then move to the Active state machine which will bring the placement group in a consistent state and handle normal operations.

Peering state machine

The placement groups rely on OSDs to exchange informations. For instance when the primary OSD receives a placement group creation message it will forward the information to the placement group who will translate it into events for the newly created state machine
The OSDs also provide a work queue dedicated to peering where events are inserted to be processed asynchronously.
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