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From "sankalp kohli (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6218) Reduce WAN traffic while doing repairs
Date Tue, 03 Dec 2013 08:28:36 GMT


sankalp kohli commented on CASSANDRA-6218:

As per my first comment, we need a way to run repair among specified endpoints. I think with
this change, you can specify data centers. So it will help people who have 3 or more DC but
not with 2 DC. 

> Reduce WAN traffic while doing repairs
> --------------------------------------
>                 Key: CASSANDRA-6218
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core, Tools
>            Reporter: sankalp kohli
>            Assignee: Jimmy MÃ¥rdell
>            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
> 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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