Why is ensemble size different from write quorum in BookKeeper?

Ensemble size (E) is the total number of bookies eligible to hold entries for a given ledger, while write quorum (Qw) is how many of those bookies each individual entry actually gets written to - Qw is always less than or equal to E.

Separating the two lets an operator stripe writes for a single ledger across more bookies than any one entry needs (for example E=5 with Qw=3), spreading load and improving read parallelism, without requiring every bookie in the ensemble to store every entry.

Ack quorum (Qa), a third related setting, is how many of the Qw bookies must confirm a write before it's considered durable - so a typical configuration like E=3, Qw=2, Qa=2 means each entry lands on 2 of 3 possible bookies and needs both to acknowledge before the write returns success.

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