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https://issues.apache.org/jira/browse/CASSANDRA-2540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13023155#comment-13023155
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Peter Schuller commented on CASSANDRA-2540:
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+1, I was discussing this just recently driven by the effects on latency.
Digest reads can definitely be useful sometimes depending on your data so I'm
not so sure about removing digest reads entirely, but certainly in many cases
it is essentially *only* detrimental (and potentially very significantly so).
This relates to the over-commit for latency purposes discussed in
CASSANDRA-2338.
> Data reads by default
> ---------------------
>
> Key: CASSANDRA-2540
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2540
> Project: Cassandra
> Issue Type: Bug
> Reporter: Stu Hood
> Fix For: 0.8.0
>
>
> The intention of digest vs data reads is to save bandwidth in the read path
> at the cost of latency, but I expect that this has been a premature
> optimization.
> * Data requested by a read will often be within an order of magnitude of the
> digest size, and a failed digest means extra roundtrips, more bandwidth
> * The [digest reads but not your data
> read|https://issues.apache.org/jira/browse/CASSANDRA-2282?focusedCommentId=13004656&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13004656]
> problem means failing QUORUM reads because a single node is unavailable, and
> would require eagerly re-requesting at some fraction of your timeout
> * Saving bandwidth in cross datacenter usecases comes at huge cost to
> latency, but since both constraints change proportionally (enough), the
> tradeoff is not clear
> Some options:
> # Add an option to use digest reads
> # Remove digest reads entirely (and/or punt and make them a runtime
> optimization based on data size in the future)
> # Continue to use digest reads, but send them to {{N - R}} nodes for
> (somewhat) more predicatable behavior with QUORUM
> \\
> The outcome of data-reads-by-default should be significantly improved
> latency, with a moderate increase in bandwidth usage for large reads.
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