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https://issues.apache.org/jira/browse/CASSANDRA-12104?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15362810#comment-15362810
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Ariel Weisberg commented on CASSANDRA-12104:
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Makes sense. I could see how a lot of the things done for writes and reads
result in fewer messages to remote DCs.
What was the coalescing window where you started to see ROI inter-dc?
I wonder if different latency or workload DCs are gong to need different
coalescing windows. I also wonder if there are cases where people abuse the
inter-dc idiom to separate things that don't actually have a lot of latency
between them.
I am still thinking through whether it's necessary to allow people to specify
the window for each DC explicitly. Seems like it might not be since the benefit
of coalescing comes from the send rate and the send rate to a remote DC is
orthogonal to the latency to that DC.
> Handle coalesce efforts for inter-dc traffic discretely from intra-dc traffic
> -----------------------------------------------------------------------------
>
> Key: CASSANDRA-12104
> URL: https://issues.apache.org/jira/browse/CASSANDRA-12104
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Thom Valley
> Priority: Minor
>
> In relationship to CASSANDRA-8692, we have discovered that pushing coalescing
> windows to the point where they have a positive impact on inter-dc traffic
> overhead appears to have causes delays in intra-dc traffic (namely, quorum
> requests between nodes). Having the same coalescing strategy apply to all
> messages (especially intra-dc request/response messages) seems like a bad
> idea.
> This was in a 5 DC environment with from 30 to 130 ms of latency between the
> DCs. Local network was entirely unrestricted 10G ethernet.
> Being able to apply different coalescing rules to those two classifications
> of traffic would allow much more effective tuning of the coalescing
> strategies, save inter-dc bandwidth while not having any impact on intra-dc
> message handling.
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