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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: How to determine if repair need to be run
Date Thu, 31 Mar 2011 16:17:58 GMT
> silly question, would every cassandra installation need to have manual repairs done on
it?
>
> It would seem cassandra's "read repair" and regular compaction would take care of keeping
the data clean.
>
> Am I missing something?

See my previous posts in this thread for the distinct reasons to run
repair. Except in special circumstances where you know exactly what
you're doing (mainly that no deletes are performed), you are
*required* to run repair often enough for GGraceSeconds:

   http://wiki.apache.org/cassandra/Operations#Frequency_of_nodetool_repair

It seems that there needs to be some more elaborate documentation
about this somewhere to point to since there seems to be confusion.

Regular compaction does *not* imply repair. Read repair only works if
(1) you touch all data within GCGraceSeconds, and (2) you touch it in
such a way that read repair is enabled (e.g., not range scans), and
(3) no node ever happens to be down, flap, or drop a request when you
touch the data in question.

Basically, unless you are really sure what you're doing - run repair.

-- 
/ Peter Schuller

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