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

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CRUSH Lookup

Any client that intends to issue reads or writes to the cluster needs to perform a lookup operation to know where the objects in question belong.

If the client doesn't have a current CRUSH map, for example when performing the first operations of a new run, it connects to one of the cluster's monitor nodes to request the latest CRUSH map. This map contains the state, topology, and configuration of the cluster. Each object is run through a fast hashing function then a bitmask is applied to ensure that the mapping function's range aligns with the key space dictated by the number of PGs in the pool. This is essentially a modulo operation. ...

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