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https://issues.apache.org/jira/browse/HDDS-15271?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ivan Andika updated HDDS-15271:
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Summary: Client should prioritize replicas covers the block BCSID (was:
Client should prioritize replicas with BCSID that cover the blocks)
> Client should prioritize replicas covers the block BCSID
> --------------------------------------------------------
>
> Key: HDDS-15271
> URL: https://issues.apache.org/jira/browse/HDDS-15271
> Project: Apache Ozone
> Issue Type: Improvement
> Reporter: Ivan Andika
> Assignee: Ivan Andika
> Priority: Major
>
> 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 preferred datanodes. 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 important things
> * We should not exclude the replicas with BCSID < N since the known container
> replica BCSID might be stale (either container location cache is stale or
> container replica heartbeat has not been recorded by SCM) and the actual
> container replica BCSID is higher. This means that we still have to read from
> replicas with BCSID < N.
> * We need to consider all BCSID >= N as being equal since both contains the
> requested blocks (even if one contains more blocks than the other). 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 read hotspot.
> We can include BCSID as the sorting requirements for client read. This should
> reduce the chances of BCSID_MISMATCH and reduce read latency.
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