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Ceph Cookbook - Second Edition by Karan Singh, Michael Hackett, Vikhyat Umrao

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How to do it…

To understand this better, let's find out the acting set for a PG from our Ceph cluster:

  1. Add a temporary object with name hosts to a pool rbd:
        # rados put -p rbd hosts /etc/hosts
  1. Check the PG name for object hosts:
        # ceph osd map rbd hosts

If you observe the output, Placement Group (0.e) has an up set [2,4,3] and an acting set [2,4,3]. So, here osd.2 is the primary OSD, and osd.4 and osd.3 are secondary and tertiary OSDs. The primary OSD is the only OSD that entertains write operations from clients. When it comes to read, by default it also comes from the primary OSD; however, we can change this behavior by setting up read ...

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