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

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Firefly – CRUSH_TUNABLES3

The Firefly profile resolves an issue where the chooseleaf CRUSH rule behavior, which is responsible for PG mappings, will come up with too few results when too many OSDs have been marked out of the cluster and will not be able to map the PG.

The following are the new tunables:

  • chooseleaf_vary_r: If a recursive chooseleaf attempt starts with a non-zero value of r, based on the number of attempts parent has already made. The default value of legacy is 0, but with such a value, CRUSH is sometimes not able to find a mapping, which can lead to PGs in an unmapped state. The optimal value (in terms of computational cost and correctness) is 1:
    • Migration impact: For the existing clusters that have lots of existing data, ...

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