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From: Jonathan Ellis <jbellis@gmail.com>
Date: Sat, 23 Jul 2011 19:04:37 -0500
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Subject: Re: do I need to add more nodes? minor compaction eat all IO
To: user@cassandra.apache.org
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On Sat, Jul 23, 2011 at 4:16 PM, Francois Richard <frichard@xobni.com> wrote:
> My understanding is that during compaction cassandra does a lot of non sequential readsa then dumps the results with a big sequential write.

Compaction reads and writes are both sequential, and 0.8 allows
setting a MB/s to cap compaction at.

As to the original question "do I need to add more machines" I'd say
that depends more on whether your application's SLA is met, than what
% io util spikes to.

-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com