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From Francois Richard <>
Subject RE: do I need to add more nodes? minor compaction eat all IO
Date Sun, 24 Jul 2011 18:18:06 GMT

Are you sure that the reads done for compaction are sequential with Cassandra 0.6.13?  This
is not what I am observing right now.  During a minor compaction I usually observe ~ 1500
to 1900 r/s while rMB/s is barely around 30 to 35MB/s.

Just asking out of curiosity.


-----Original Message-----
From: Jonathan Ellis [] 
Sent: Saturday, July 23, 2011 5:05 PM
Subject: Re: do I need to add more nodes? minor compaction eat all IO

On Sat, Jul 23, 2011 at 4:16 PM, Francois Richard <> 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

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

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