Repair: compare all trees together (for a given range/cf) instead of by pair in
isolation
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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
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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