[jira] [Commented] (CASSANDRA-3200) Repair: compare all trees together (for a given range/cf) instead of by pair in isolation

["Peter Schuller (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-3200?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13104324#comment-13104324 ] 

Peter Schuller commented on CASSANDRA-3200:
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This is definitely an interesting idea. But FWIW, I think it is more important to make repair be more incremental/less bulky/more continuous than it is to be efficient in terms of absolute amount of data transfered. I wonder to what extent an implementation of this ticket might be obsoleted by a solution to CASSANDRA-2699 (not that the desire to not transfer things unnecessarily goes away, but in terms of the implementation details).

> Repair: compare all trees together (for a given range/cf) instead of by pair in isolation
> -----------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-3200
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3200
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>            Priority: Minor
>              Labels: repair
>             Fix For: 1.0.1
>
>
> Currently, repair compare merkle trees by pair, in isolation of any other tree. What that means concretely is that if I have three node A, B and C (RF=3) with A and B in sync, but C having some range r inconsitent with both A and B (since those are consistent), we will do the following transfer of r: A -> C, C -> A, B -> C, C -> B.
> The fact that we do both A -> C and C -> A is fine, because we cannot know which one is more to date from A or C. However, the transfer B -> C is useless provided we do A -> C if A and B are in sync. Not doing that transfer will be a 25% improvement in that case. With RF=5 and only one node inconsistent with all the others, that almost a 40% improvement, etc...
> Given that this situation of one node not in sync while the others are is probably fairly common (one node died so it is behind), this could be a fair improvement over what is transferred. In the case where we use repair to rebuild completely a node, this will be a dramatic improvement, because it will avoid the rebuilded node to get RF times the data it should get.

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