thetumbled commented on code in PR #4258:
URL: https://github.com/apache/bookkeeper/pull/4258#discussion_r1582090916
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site3/website/src/pages/bps/BP-66-support-throttling-for-zookeeper-read-of-rereplication.md:
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+# BP-66: support throttling for zookeeper read of rereplication
+
+### Motivation
+
+Each time the cluster trigger the re-replication, all replicators will read
data from zookeeper. This can cause a great pressure on Zookeeper. We need to
support throttling for zookeeper read of re-replication.
+
+For example, in a Pulsar cluster, we enable auto-recovery for every bookie.
There are 400 bookies in a cluster, which means there are 400 replicators in
the cluster.
+And there are about 3000 ledgers in each bookie, 1/3 of them are small
ledgers, whose size is less than 0.1MB, that is 1000 small ledgers in each
bookie.
+If we decommission one bookie, the read latency of zookeeper will increase to
minutes.
+
+
+### Configuration
+add the following configuration:
Review Comment:
- ZK can't scale out. As the Pulsar & Bookkeeper cluster scale out, the
number of replicators must scale out inevitably. When the number of replicators
reach to tens or hundreds, the latency will soar to unacceptable level.
- For the convenience of operation and maintenance, we always set
`autoRecoveryDaemonEnable=true` for every bookie. We do not adopt the other two
options:
- Deploying another cluster for AutoRecovery increase the complexity of
the whole system.
- Make small part of bookies in the Bookkeeper cluster work as replicator
and set a high value of `replicationRateByBytes` is dangerous. because the
throughput of normal client will be impacted by the replication throuhput.
- Actually, there is not a way to relieve the pressure of zk in replicatioin
currently.
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