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Date: Tue, 21 Apr 2015 13:05:00 +0000 (UTC)
From: "Jonathan Ellis (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (CASSANDRA-7168) Add repair aware consistency
 levels
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    [ https://issues.apache.org/jira/browse/CASSANDRA-7168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14504910#comment-14504910 ] 

Jonathan Ellis commented on CASSANDRA-7168:
-------------------------------------------

I'm fine with adding this as opt-in.  But in the meantime I do think we should drop digest reads.  It's an optimization that really doesn't buy us much, at the cost of a lot of complexity (that I'm still not entirely sure is bug-free) as well as significantly complicating this new optimization (that has much more potential to actually save work).

> Add repair aware consistency levels
> -----------------------------------
>
>                 Key: CASSANDRA-7168
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7168
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: T Jake Luciani
>              Labels: performance
>             Fix For: 3.1
>
>
> With CASSANDRA-5351 and CASSANDRA-2424 I think there is an opportunity to avoid a lot of extra disk I/O when running queries with higher consistency levels.  
> Since repaired data is by definition consistent and we know which sstables are repaired, we can optimize the read path by having a REPAIRED_QUORUM which breaks reads into two phases:
>  
>   1) Read from one replica the result from the repaired sstables. 
>   2) Read from a quorum only the un-repaired data.
> For the node performing 1) we can pipeline the call so it's a single hop.
> In the long run (assuming data is repaired regularly) we will end up with much closer to CL.ONE performance while maintaining consistency.
> Some things to figure out:
>   - If repairs fail on some nodes we can have a situation where we don't have a consistent repaired state across the replicas.  
>   


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