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There is a rule of thumb for how many ledgers per bookie. Main constraint is ZK for all I know, but this is solved by scaling its machines (not increasing number of machines). @Penghui probably as more details

Yes, the major limitation is Zookeeper for managing huge amounts of topic metadata and Ledger metadata. This is why we build https://github.com/streamnative/oxia (A horizontally scalable metadata service)

Hadn't seen that before. Weill give it a look and keep an eye on it.