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From "Jacek Lewandowski (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-5972) Reduce the amount of data to be transferred during repair
Date Tue, 03 Sep 2013 14:48:51 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13756667#comment-13756667
] 

Jacek Lewandowski edited comment on CASSANDRA-5972 at 9/3/13 2:47 PM:
----------------------------------------------------------------------

Yes, that does assume a bunch of things. I know that it is far more complicated than the current
solution. And, I agree that a huge advantage of the solution proposed in https://issues.apache.org/jira/browse/CASSANDRA-3200
is its rather straightforward implementation, especially in comparison to the one described
here. 

However, I've found it difficult to prove that the my solution is worse :)

For example, in a situation where one of n+1 replicas has outdated sub-range R (and the rest
are in sync), the procedure from C*-3200 will initiate n+1 data transfers. With the algorithm
described here, we would require only one data transfer with a probability (n-1)/n, or 2 data
transfers with a probability 1/n. It depends on which node starts the sequence.  

                
      was (Author: jlewandowski):
    Yes, that does assume a bunch of things. I know that it is far more complicated than the
current solution. And, I agree that a huge advantage of the solution proposed in https://issues.apache.org/jira/browse/CASSANDRA-3200
is its rather straightforward implementation, especially in comparison to the one described
here. 

However, I've found it difficult to prove that the my solution is worse :)

For example, in a situation where one of n+1 replicas has outdated sub-range R, the procedure
from C*-3200 will initiate n+1 data transfers. With the algorithm described here, we would
require only one data transfer with a probability (n-1)/n, or 2 data transfers with a probability
1/n. It depends on which node starts the sequence.  

                  
> Reduce the amount of data to be transferred during repair
> ---------------------------------------------------------
>
>                 Key: CASSANDRA-5972
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5972
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Jacek Lewandowski
>            Priority: Minor
>
> Currently, when a validator finds a token range different in n replicas, data streams
are initiated simultaneously between each possible pair of these n nodes, in both directions.
It yields n*(n-1) data stream in total. 
> It can be done in a sequence - Replica[1] -> R[2], R[2] -> R[3], ... , R[n-1] ->
R[n]. After this process, the data in R[n] are up to date. Then, we continue: R[n] -> R[1],
R[1] -> R[2], ... , R[n-2] -> R[n-1]. The active repair is done after 2*(n-1) data transfers
performed sequentially in 2*(n-1) steps.

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