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From "Stu Hood (JIRA)" <j...@apache.org>
Subject [jira] [Issue Comment Edited] (CASSANDRA-2540) Data reads by default
Date Fri, 22 Apr 2011 07:48:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13023129#comment-13023129
] 

Stu Hood edited comment on CASSANDRA-2540 at 4/22/11 7:46 AM:
--------------------------------------------------------------

Also, I targeted this at 0.8 because the "digest but not data" problem feels far too much
like a bug, and the potential latency improvement is huge.

EDIT: Also, I suspect that removing digest reads entirely would hugely simplify the read path.

      was (Author: stuhood):
    Also, I targeted this at 0.8 because the "digest but not data" problem feels far too much
like a bug, and the potential latency improvement is huge.
  
> 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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