Ivan Andika created HDDS-15271:
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Summary: Client should prioritize replicas with BCSID covering the
blocks.
Key: HDDS-15271
URL: https://issues.apache.org/jira/browse/HDDS-15271
Project: Apache Ozone
Issue Type: Improvement
Reporter: Ivan Andika
Assignee: Ivan Andika
Currently, client read prioritizes based on the locality, datanode status
(maintenance & decommission), etc. However, the client do not check whether the
replica BCSID covers the block the client is trying to read. This causes
BCSID_MISMATCH which triggers failover and increases read latency.
The idea of this patch is to also consider the BCSID as a hint (not a
requirement) for client to pick a datanode. If a client requested a block with
BCSID N, any datanodes that contains BCSID >= N should be prioritized over
those that have datanodes BCSID < N.
However, we need to note a few things
* We should not exclude the replicas with BCSID < N since the container replica
BCSID might be stale (either container location cache is stale or container
replica heartbeat has not been recorded by SCM). This means that although we
will read from replicas with BCSID < N if the previous replicas with BCSID >= N
have been replicated.
* We need to consider all BCSID >= N as being equal. So a replica 1 with BCSID
N + 1 and replica 2 with BCSID N + 2 are the same even though replica 2 is more
up-to-date. This should prevent hotspot.
We can include BCSID as the sorting requirements for client read.
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