Stephen O'Donnell created HDDS-8505:
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Summary: ReplicationManager: Add configurable global replication
limit
Key: HDDS-8505
URL: https://issues.apache.org/jira/browse/HDDS-8505
Project: Apache Ozone
Issue Type: Sub-task
Components: SCM
Reporter: Stephen O'Donnell
Assignee: Stephen O'Donnell
We should make it possible to configure a global replication limit, limiting
the number of inflight containers pending creation. A larger cluster would be
capable of having more inflight replication than a smaller cluster, so the
limit should be a function of the number of datanodes on the cluster, and the
limit of the number of commands which can be queued per datanode and some
weighting factor.
For example, if each datanode can queue 20 replication commands, and there are
100 nodes in the cluster, then the natural limit is 20 * 100. However, that
assumes that commands are queued evenly across all datanodes, which is
unlikely. With a global limit we would prefer that all datanodes are not fully
loaded with replication commands simultaneously, so we may want to impose a
limit of half that number, with a factor of 0.5, eg 20 * 100 * 0.5 = 1k pending
replications.
At one extreme this would result in all datanodes in the cluster having half
their maximum tasks queued, but in practice, some DNs are likely to be at their
limit while others have zero or less than half queued.
If the limits were perfectly defined, such that in a single heartbeat a
datanode can complete all its queued work just at the end of the heartbeat
interval, then reducing the number of queued commands by half would make the
datanode busy for only half its heartbeat interval. As the datanodes will all
heartbeat at different times, all the busy and non-work periods across all the
datanodes would combine in a load profile that would show some datanodes are
always idle, reducing the overall load on the cluster.
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