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

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Bobtail – CRUSH_TUNABLES2

The Bobtail profile fixes several CRUSH issues:

  • In CRUSH hierarchies with a smaller number of devices in buckets, such as a host leaf bucket with one OSDs - three OSDs under it, the PGs may get mapped to less than the desired number of replica
  • In larger Ceph clusters with several hierarchy layers (row, rack, host, osd) it is possible that a small amount of PGs could get mapped to less than the desired amount of OSDs
  • If an OSD gets marked out in Bobtail, the data usually gets rebalanced to nearby OSDs in the bucket instead of across the entire CRUSH hierarchy

The following are the new tunables:

  • choose_local_tries: The number of local retries is given by this tunable. The legacy and optimal values are 2 and 0

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