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https://issues.apache.org/jira/browse/CASSANDRA-6218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13836376#comment-13836376
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Jimmy Mårdell commented on CASSANDRA-6218:
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Sorry, I should have been cleared that my patch didn't match the original
ticket precisely.
The patch addresses the case when you have many DC's in your cluster, for
instance two in the US (A,B) and two in Europa (C,D). Then it's nice to do
pairwise repair between DC's A-B and C-D and then B-C and the finally A-B,C-D
again.
> Reduce WAN traffic while doing repairs
> --------------------------------------
>
> Key: CASSANDRA-6218
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6218
> Project: Cassandra
> Issue Type: New Feature
> Components: Core, Tools
> Reporter: sankalp kohli
> Assignee: Jimmy Mårdell
> Priority: Minor
> Fix For: 2.0.4
>
> Attachments: trunk-6218-v2.txt, trunk-6218-v3.patch, trunk-6218.txt
>
>
> The way we send out data that does not match over WAN can be improved.
> Example: Say there are four nodes(A,B,C,D) which are replica of a range we
> are repairing. A, B is in DC1 and C,D is in DC2. If A does not have the data
> which other replicas have, then we will have following streams
> 1) A to B and back
> 2) A to C and back(Goes over WAN)
> 3) A to D and back(Goes over WAN)
> One of the ways of doing it to reduce WAN traffic is this.
> 1) Repair A and B only with each other and C and D with each other starting
> at same time t.
> 2) Once these repairs have finished, A,B and C,D are in sync with respect to
> time t.
> 3) Now run a repair between A and C, the streams which are exchanged as a
> result of the diff will also be streamed to B and D via A and C(C and D
> behaves like a proxy to the streams).
> For a replication of DC1:2,DC2:2, the WAN traffic will get reduced by 50% and
> even more for higher replication factors.
>
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